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.
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.
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.
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.
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.
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.
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.
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.
The strike could well affect the morning flights.
An enhanced possibility, not a confirmed result.
Your package may arrive after the weekend.
Your package may arrive after the weekend.
We allow for this arrival time.
Trains can be busy on Friday evenings.
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.
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 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 pharmacy might not be open on public holidays.
The speaker allows for this opening schedule.
The queue shouldn't be long at this hour.
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.
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.
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 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.