Degrees of Certainty

Must shows that the speaker is almost certain of a conclusion; can’t/cannot shows that they reject a possibility. This is an assessment based on information, not proof of truth. Distinguish deduction from obligation and keep the base form after the modal.

Published October 3, 2026

Must / Can’t: confident deductions about the present

1. The speaker draws a conclusion

Must in She must be tired conveys a confident deduction: the speaker is almost certain she is tired. Can’t in She can’t be at home shows that the speaker regards that possibility as impossible. Reality may turn out differently: the grammar conveys the speaker’s assessment.

The evidence may be stated in the same sentence, have been heard earlier, or be apparent from the situation. You do not need to add because every time. Even a short You must be tired can be a natural response to an account of a long journey.

You've been travelling all day. You must be tired.

The evidence is given before the deduction; the tiredness has not been checked directly.

That can't be our train — its destination is different.

The speaker rejects a possibility based on the available information.

The kettle is still warm. Someone must have used it recently.

The warm kettle points to an earlier action, rather than proving that the person is still here. Must have used touches on the neighbouring topic of past deduction.

2. A modal is followed by the base form

After must and can’t, use the base form without to: must know, can’t belong, must be. Do not add -s because of he/she/it. Must and can’t themselves do not change for person either.

Do not add do/does to a modal. For an ordinary question about a possible explanation, Could this be…? or Do you think this could be…? is useful. A question with Must…? has its own uses, but does not require does.

How to build it

subject + must / can’t (cannot) + base form: must be, can’t know

subject / S: The grammatical subject: who or what the clause is about; the verb agrees with it. In The book was written, the book is the subject, although someone else does the action.
V1 / base form: The base verb without to: work, go, be.

This parcel must belong to our neighbour — his name is on the label.

After must, use belong, not belongs.

She can't know the new password yet — we haven't sent it to her.

Can’t know confidently rejects the possibility.

Could this be the wrong entrance?

The question checks a possibility; it does not yet assert a confident conclusion.

3. Deduction and obligation are different meanings

Must can also express necessity: You must wear a badge. Distinguish the meanings by intention: is the speaker imposing a requirement, or explaining what they believe on the basis of information?

Do not determine the meaning solely by present or future time. A rule may apply now; a deduction may concern a state or a habitual routine. The main focus of this lesson is deductions about the present.

All visitors must wear a badge.

A requirement for visitors, not a guess about what they are wearing.

You must be relieved now that the work is finished.

A deduction about the listener’s state; there is no command to feel relieved.

4. Can’t, mustn’t and don’t have to

To express “I am certain this is not the case”, this lesson practises can’t/cannot: That can’t be right. Cannot is the full form; it may be suitable in writing or for emphasis.

Mustn’t in You mustn’t leave your bag here expresses prohibition. Do not therefore automatically replace can’t with mustn’t. Modal meanings depend on context: can’t can also express prohibition or inability.

Don’t have to usually expresses a lack of necessity, while doesn’t have to be can reject the inevitability of an explanation: The parcel doesn’t have to be from the shop means it is not necessarily from the shop. It does not assert that it is definitely not from there.

This can't be the right platform — the board shows a different train.

A deduction from the information on the board.

You mustn't leave your bag on the platform.

Prohibition.

You don't have to book a seat on this line.

No obligation to reserve a seat.

5. A state, habit or process

For a state or habit, use a simple form: must know, must live, must open early. For a current process, use must/can’t + be + -ing. Be remains in its base form after the modal.

Know in the sense of possessing knowledge is not normally used in the continuous: must know the code. Choose the form according to the verb’s meaning, not whether the action can be seen: the invisible process thinking can also take -ing.

How to build it

must / can’t + be + -ing: must be working

V-ing / -ing: The verb form ending in -ing: working, going. Its role depends on the construction.

I can hear someone chopping vegetables. They must be preparing dinner.

The speaker draws a conclusion about a current process; another explanation is still possible.

They can't be sleeping — I can hear them talking.

The stated evidence directly contradicts the possibility of sleeping.

The queue is already long at seven. This bakery must open early.

A deduction about the usual opening time.

6. Other ways to express certainty

Have/has (got) to can also express a confident deduction: There has to be an explanation. This use is particularly common in North American English and also occurs in British English. Do not regard must as exclusively written or have to as exclusively conversational.

When you want to leave a possibility open, choose might, may or could. This is a difference in stated confidence, not a scale of exact percentages. Evidence may support a conclusion without excluding every other explanation.

For a deduction about an earlier event, use must/can’t have + V3. In your own answer, preserve the required time and do not present an assumption as a verified fact. I imagine and surely can also change the tone and strength of an utterance, depending on the situation.

There has to be an explanation for these numbers.

Certainty can be expressed with has to be or must be.

The shutters are down, so the café must be closed.

The speaker sees this as the best explanation; the sentence does not prove that an error is impossible.

The café might be closed. Let’s check its opening hours.

An open possibility with less stated confidence; checking is appropriate even after a stronger deduction.

Common mistakes

Incorrect:This parcel must belongs to our neighbour — his name is on the label.

Correct:This parcel must belong to our neighbour — his name is on the label.

After must, the verb takes its base form: the third-person -s ending is removed when the modal is present.

Incorrect:The alarm can't to be working — nobody reacted to the smoke.

Correct:The alarm can't be working — nobody reacted to the smoke.

After can’t, as after must, there is no to: the base verb follows directly.

Incorrect:She must is waiting at the wrong entrance — her phone shows the north gate.

Correct:She must be waiting at the wrong entrance — her phone shows the north gate.

After the modal, use base be, not finite is; subject agreement is not expressed there.

Incorrect:They don't must be at home — both bikes are gone.

Correct:They can't be at home — both bikes are gone.

Must does not combine with don’t. To confidently reject the possibility that they are at home, this pattern requires can’t. The missing bikes support the speaker’s deduction; they are not infallible proof.

Incorrect:He must be knowing the code — he opened the door in one second.

Correct:He must know the code — he opened the door in one second.

Know in the sense of knowing the code normally describes a state, so use must know here. For a process with a different verb, must be + -ing is possible.

Incorrect:That has to being our taxi — the driver is waving at us.

Correct:That has to be our taxi — the driver is waving at us.

Has to is followed by base be; being is not possible here.

Incorrect:Nobody is answering the doorbell, so the flat must to be empty.

Correct:Nobody is answering the doorbell, so the flat must be empty.

After modal must, use base be without to. Must be expresses the speaker’s confident deduction, not a guarantee that it is correct.

Sources

Might / Could / May for possibility

Why we need the middle of the scale

When you do not want to present an unverified explanation as fact, use might, could or may. They let you say “this is possible”, leaving room for other explanations.

The choice shows the speaker’s assessment based on available information and context. One person may be confident, while another knows less or deliberately speaks more cautiously. A modal by itself does not establish an event’s objective probability.

First decide whether you are presenting a fact, a confident deduction or only a possible explanation. Might, could and may are common ways to express the third case.

Daniel might be in the meeting room.

The speaker can see only a closed door. This is one possibility, not an established location.

The delay could be a network problem.

Could presents one of several plausible causes and leaves room for others.

Some services may be unavailable tonight.

An official notice honestly warns of a possibility without promising an outage.

Form: a modal plus the base form

Might, could and may are modal verbs. They take the base form: no third-person -s, no to, and no was, has or had. The subject does not change the modal: I might, she might, they might.

Form a negative with not immediately after the modal: might not, may not. Form a question through inversion: Could this be…? Auxiliary do is neither needed nor used here.

Use be for a state, the verb itself for an action, and be + -ing for something that may be happening right now. Have after a modal also takes its base form; with present possession it simply means “possess”, rather than being part of a perfect construction.

How to build it

subject + might / could / may (+ not with might/may for “possibly not”) + base form (be / have / V / be + V-ing)

subject / S: The grammatical subject: who or what the clause is about; the verb agrees with it. In The book was written, the book is the subject, although someone else does the action.
verb / V: A word for an action or state, such as work or be. The pattern determines its form.
V1 / base form: The base verb without to: work, go, be.
V-ing / -ing: The verb form ending in -ing: working, going. Its role depends on the construction.

Marko might know the door code.

After might, use know, not knows: the modal does not allow a third-person ending.

She might have a spare key in her bag.

Have here is ordinary possession in its base form. Might has is impossible.

They could be sitting in the garden.

Be + -ing shows an action that, in the speaker’s view, may be in progress right now.

It may rain before the evening.

There is no to between may and rain: modals combine directly with the base form.

Three words with roughly the same meaning

In many affirmative sentences about possibility, might, could and may express similar meanings. None specifies an exact percentage of probability. However, do not mechanically extend this interchangeability to every meaning, negative or question.

The difference worth noticing concerns tone and register. May sounds more official and is natural in notices, instructions and customer correspondence. Might can sound the most cautious of the three in conversation. Could often presents an explanation as one of several options worth checking.

These are tendencies, not strict rules. If a task asks for a particular modal, practise that one. In your own text, choose a natural option according to meaning and tone.

The report might be ready by Thursday.

A cautious conversational possibility: the speaker clearly makes no commitment.

Delivery may take longer than usual this week.

May sounds natural in an official message to customers.

The noise could be coming from the ventilation.

Could offers one explanation that still needs checking.

Now, right now and what lies ahead

This construction covers three time perspectives, none of which requires a different modal. A state now: might be + place, state or characteristic. An action right now: might be + -ing. A future event: might + verb with a future time expression.

Will often expresses a prediction with greater stated confidence and can also express a decision or promise. Might, could and may leave the possibility open. It will rain and It might rain therefore differ in the strength of the prediction.

Tomorrow, later and next week help indicate the future. Time may also be clear from the conversation: you do not have to add a separate marker to every sentence.

The office may be closed at lunchtime.

A current or recurring state: the speaker has not checked the schedule.

Anna could be waiting downstairs right now.

Be + -ing ties the guess to this moment.

The road works might continue until the end of the month.

A future event; the time expression makes the future reference explicit without changing the form.

Negatives: “possibly not” versus “certainly not”

The negative possibility forms are might not and may not. They mean “perhaps this will not happen” and retain the same uncertainty as the affirmative. The speaker simply considers the opposite outcome.

Can’t and couldn’t in their impossibility meaning reject an explanation, whereas might not/may not merely allow a negative possibility. That can’t be her number and That might not be her number express different confidence levels. Information can be checked after either one: the speaker can also be wrong.

For ordinary “possibly not”, put not after the modal: might not be, may not know. Negation can also belong to a separate phrase; do not move it mechanically, since that can change the emphasis or meaning.

How to build it

might not / may not + base form = possibly not · can’t (couldn’t) + base form = certainly not

V1 / base form: The base verb without to: work, go, be.

The parcel might not arrive today.

The speaker considers a negative outcome without asserting it.

They may not know about the change of time.

Not comes between may and the base form know.

There might not be enough seats for everyone.

In a there + be construction, not also goes immediately after the modal.

Intensifiers: when an explanation becomes very plausible

Sometimes a speaker wants to show that an explanation is not merely possible but quite likely, without moving to a confident deduction. To do this, put an adverb after the modal: might well, may well, could easily, could well.

Well after a modal makes the situation sound noticeably more likely. Could easily works similarly and is particularly natural with duration, cost or quantity: “this could easily take three weeks”.

The usual pattern for “quite possibly” is might/may/could + well + verb. In could easily last, easily also strengthens possibility rather than describing how easy a task is. Adverb position depends on meaning; could be easily repaired, for example, can mean that repairing it would be easy.

How to build it

might / may / could + well / easily + base form

V1 / base form: The base verb without to: work, go, be.

The delay could easily last another hour.

Easily increases the expressed likelihood, but the sentence remains an assessment, not an announcement.

The library might well be closed by six.

Well between might and be strengthens the possibility: “quite possibly”.

This invoice may well be a mistake.

The more official register of may combines well with the intensifier.

Questions and short responses

For an ordinary question about a possible explanation, use Could this be…? Questions with might are often more formal. May in direct questions most often concerns permission, so could is usually more convenient for neutrally checking an explanation.

The most common conversational way to ask for someone else’s guess is to embed it in a question with do you think: Do you think she might come? The order inside remains declarative, without inversion.

In a short answer, you can leave just the modal: It might, They could. After a question with be, retaining be is natural: She might be. Depending on context, shorter She might is also possible; repeating be is not an absolute requirement.

Could this be another network problem?

Inversion with could is a standard way to ask about a possible explanation.

Do you think Andriy might join the review?

After do you think, the order is declarative: Andriy might join.

— Will the shop still be open? — It might be.

The short answer repeats the modal and be, omitting the rest.

Scope: what is not covered here

The strong ends of the scale — must for a confident deduction and can’t for confident rejection — belong to a neighbouring rule in the same topic. They appear here only as a contrast, showing when the speaker regards the evidence as sufficient and when not.

Guesses about past events with might/may/could have + V3 are covered separately. Here we practise a possible present state, current process or future event.

Three other neighbouring uses are easy to confuse because of their form. May for permission (You may leave now) is another function of the same word. Can for general possibility (Trains can be busy on Fridays) describes what happens in general, not one specific case. Could for past ability (I could swim at five) is also unrelated to guessing. Context shows which meaning applies each time.

That must be the courier; nobody else has the gate code.

The speaker regards this information as sufficient for a confident deduction; must itself does not guarantee that the deduction is correct.

Trains can be busy on Friday evenings.

Can describes a general pattern, not a guess about a particular train today.

You may leave your bag at the reception.

The same may, but a different function: permission, not an assessment of probability.

Common mistakes

Incorrect:She might has a spare key in her bag.

Correct:She might have a spare key in her bag.

A modal always takes the base form. Has is impossible here, whether as ordinary possession or as part of another construction.

Incorrect:It may to rain before the evening.

Correct:It may rain before the evening.

There is no to between the modal and verb: may rain, not may to rain.

Incorrect:He could knows the door code.

Correct:He could know the door code.

The third person does not add -s after a modal: the modal occupies the finite verb position.

Incorrect:There might not is enough space for everyone.

Correct:There might not be enough space for everyone.

After might not, use base be, not is.

Incorrect:I might well to send the draft today.

Correct:I might well send the draft today.

The intensifier well does not introduce a to-infinitive: it is still followed by the base form without to.

Incorrect:Do you think she might comes to the review?

Correct:Do you think she might come to the review?

Inside a question with do you think, word order is declarative, but the modal still requires the base form.

Sources

How to Express Degrees of Certainty

1. The Speaker’s Assessment, Not a Probability Measurement

Modal words help show how you assess a situation. Must presents a strong conclusion, can’t rejects a possibility, should/ought to expresses expectation, and may/might/could allows for a possibility.

Do not attach numbers such as 95% or 50% to them. Context, intonation, and other words affect the strength of the message. Even a confident conclusion may turn out to be wrong.

How to build it

Confident conclusion: must · expectation: should/ought to · possibility: may/might/could · rejecting a possibility: can’t

The lift must be out of order — there is a repair sign on it.

The lift is probably not working — there is a repair sign on it.

The speaker makes a confident conclusion from an available sign.

The lift might be out of order. I haven’t checked.

The lift might not be working. I haven’t checked.

The same possibility is presented more cautiously.

The lift should be repaired by noon, if everything goes to plan.

If everything goes to plan, the lift should be repaired by noon.

The expectation is based on a plan; it does not confirm that the repair has been completed.

2. Must and Can’t: Strong Conclusions

Must shows that the speaker considers a particular explanation very convincing. Can’t/cannot shows that the speaker considers a possibility impossible; couldn’t can also have this meaning. The basis may be stated nearby or already be clear from the conversation.

In this lesson, we practise can’t for rejecting a possibility. Mustn’t in the prohibition You mustn’t open this door has a different function. Always check whether the person is making a conclusion or setting a rule.

How to build it

must / can’t + base form: must be, can’t belong

V1 / base form: The base verb without to: work, go, be.

Their suitcases are gone and the key is back, so the guests must be away.

Their suitcases are gone and the key has been returned, so the guests must be away.

Several signs support the conclusion; the form still expresses the speaker’s assessment.

That can't be our suitcase — ours is red.

The speaker rejects the possibility by comparing a feature.

You mustn't open this door during the exam.

A prohibition, not an assessment of probability.

3. Should and Ought to: Well-Founded Expectation

Should and ought to can mean “we expect this to happen” based on a schedule, experience, or known conditions. This is not a guarantee. In these sentences, they do not give advice, although that function is also possible in other contexts.

Shouldn’t expresses the expectation that something will not happen: It shouldn’t take long. After should, use the base form without to; in this pattern, use ought together with to. For unwanted predictions, might or is likely to often sounds more natural.

The post should come before ten — it normally does.

An expectation based on the usual schedule.

The film ought to be good; the reviews are excellent.

An expectation based on the reviews.

The repair shouldn't take more than an hour.

A prediction about duration, not a prohibition against the technician working longer.

4. May, Might, Could: An Open Possibility

These words often express a similar meaning of possibility. May can sound more formal; might is often more cautious. There is no fixed numerical scale that allows you to rank them unambiguously in every sentence.

May/might/could well strengthens plausibility: “it is quite possible.” But the possibility remains a possibility. Can in an ordinary affirmative statement often describes general possibility, so for a specific guess, He might be at home is more natural than He can be at home.

The strike could well affect the morning flights.

An enhanced possibility, not a confirmed result.

Your package may arrive after the weekend.

We allow for this arrival time.

Trains can be busy on Friday evenings.

Can describes something that sometimes happens, not a specific guess about today.

5. Negation Changes the Meaning in Different Ways

Might not/may not means “perhaps not.” Can’t, when used for a conclusion, rejects a possibility. Shouldn’t conveys the expectation that an event or feature will be absent. First decide exactly what you want to negate.

Do not consider could not a simple equivalent of “perhaps not”: when it means impossibility, it is stronger. Other functions of modal words, such as permission or ability, are determined separately by the context.

How to build it

can’t be — I consider it impossible · might not be — perhaps it is not there · shouldn’t be — I expect it not to be there

The pharmacy can't be open — the door is locked and the sign says closed.

The speaker confidently rejects the possibility.

The pharmacy might not be open on public holidays.

The speaker allows for this opening schedule.

The queue shouldn't be long at this hour.

An expectation that may not come true.

6. Combine the Assessment with the Context

New information can change your confidence. An addition such as but I’m not completely sure can soften a strong conclusion; this is not automatically a grammatical contradiction. However, for a simple neutral assumption, might is often clearer.

Distinguish confidence in an event from confidence in a possibility: I’m certain she will win states confidence that she will win, while I’m certain she could win expresses certainty that she has a chance. Do not replace one with the other without changing the meaning.

For earlier events, use modal + have + V3, while the main focus here is present states and future events. Add a time reference if it is needed for understanding.

The report might be ready tomorrow, but nobody has confirmed it.

An open possibility with an addition indicating a lack of confirmation.

She must be the owner, though I'm not completely sure.

A strong initial conclusion is softened; the speaker does not claim infallibility.

The courier said he would come in the morning, so he should be here soon.

The expectation is based on an arrangement.

Common mistakes

Incorrect:He must to be the new teacher.

Correct:He must be the new teacher.

After must, use the base form without to.

Incorrect:They should to arrive soon.

Correct:They should arrive soon.

After should, as after other modal verbs, use the base form without to.

Incorrect:I don't know where he is — he could is in the garden.

Correct:I don't know where he is — he could be in the garden.

After the modal could, use the base form be, not is. Could be here presents a possible explanation.

Incorrect:It cannot is Tom’s coat — his is blue.

Correct:It cannot be Tom’s coat — his is blue.

After cannot, use the base form be, not is.

Incorrect:The train should being here by now.

Correct:The train should be here by now.

After a modal word, use the base form be, not being.

Incorrect:She ought be here soon.

Correct:She ought to be here soon.

In this pattern, use ought with to, unlike should: ought to be, but should be.

Sources

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