1. The basic idea: regulating action rather than describing a fact
A deontic statement guides behaviour: it tells someone what they must, may, may not or need not do. Its source may be a law, contract, official, the speaker's own decision or an accepted norm.
Interpret each form in context. In The server must be offline, must may express a conclusion based on evidence. In The server must be taken offline before maintenance, it sets a requirement. The presence of a rule, permission or control over an action makes the second reading deontic.
authority / norm / speaker → required, permitted, forbidden or optional action
All contractors must display an identity badge.
All contractors must display an identity badge.
Must conveys the organisation's requirement governing contractors' behaviour.
The lights are out, so the office must be empty.
The lights are out, so the office must be empty.
This is an epistemic deduction about a probable situation, not a rule regulating someone's action.
2. Strong obligation and the source of a requirement
Must and have to can both express strong obligation. Must often presents a requirement imposed by the speaker or authority issuing a rule; have to naturally reports an external requirement or practical necessity. This is a tendency, not an absolute test: in an official rule, the document's author is also the source of must.
Need to also expresses necessity, but does not always emphasise an order. Specify the source in the context — a contract, policy, law or instruction — because otherwise several forms may sound natural.
Shall can establish a formal obligation in contracts, legislation and official provisions. However, editorial conventions and legal effects vary. Do not mechanically substitute shall for must or interpret it without the document's context.
must / have to / need to + base verb
- 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.
Under the grant agreement, we have to retain the records for six years.
Under the grant agreement, we have to retain the records for six years.
Have to reports an external contractual requirement.
You must disclose any conflict of interest before the interview.
You must disclose any conflict of interest before the interview.
An authorised person or document directly establishes a rule.
The licensee shall notify the authority within five working days.
The licensee shall notify the authority within five working days.
Within this formal document, shall establishes a contractual obligation. Outside this genre, it is not a universal replacement for must.
3. Permission: asking, granting and describing
Can and may can both ask for and grant permission. May is typical of formal policies and official permission; can is more neutral and very common in conversation. Could naturally softens a request for permission, but the reply normally grants permission with can or may rather than could.
Be allowed to and be permitted to are useful for past or future reference and for institutional rules expressed in the passive. They do not automatically make a statement more polite: register depends on the complete text and situation.
Can/May I + base verb?; subject + can/may + base verb; be allowed/permitted to + base verb
- 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.
Researchers may consult the archive by appointment.
Researchers may consult the archive by appointment.
May expresses formal permission together with the condition under which it applies.
Could I take the original file off site? — No, but you may request a digital copy.
Could I take the original file off site? — No, but you may request a digital copy.
Could softens the request; may in the reply grants specific permission.
4. Prohibition is not the same as no obligation
Must not/mustn't, cannot/can't and formal may not can express prohibition: the action is not permitted. Must not often sounds like a rule or direct instruction; can't naturally reports a restriction; may not formally denies permission in a regulatory text.
Don't have to, don't need to and needn't mean something different: an action is not required, but remains possible. You mustn't attend prohibits attendance, whereas You don't have to attend leaves a choice.
prohibition: must not / cannot / may not; no obligation: do not have to / do not need to / need not
Visitors must not photograph unpublished manuscripts.
Visitors must not photograph unpublished manuscripts.
Must not prohibits the action. It is more than advice to avoid it.
You don’t have to print the form; a digital copy is acceptable.
You don’t have to print the form; a digital copy is acceptable.
Printing is optional, not forbidden.
5. Advice, weaker obligation and the force of a statement
Should and ought to often present an action as desirable, sensible or expected under a norm. Without supporting context, they do not turn it into a legal requirement. Ought to is more formal and less common, particularly in questions and negatives.
Do not assign fixed strength percentages to modal forms. Must in a friendly invitation, should in a professional code of ethics and can't in an official's reply derive their force from the speaker's role, genre, consequences and surrounding wording.
should + base verb; ought to + base verb
- 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.
Reviewers should explain any substantial change they recommend.
Reviewers should explain any substantial change they recommend.
Should expresses a professional recommendation rather than an explicitly enforced requirement.
Applicants must submit the declaration; they should also keep a copy.
Applicants must submit the declaration; they should also keep a copy.
Must identifies a requirement of the procedure; should gives useful advice.
6. Time: past and future obligation
For an actual obligation in a completed past situation, the standard form is had to. For past permission, use was/were allowed to. Do not mechanically carry present must into a narrative about a requirement fulfilled yesterday.
Will have to naturally expresses a specific future obligation. Must can refer to a future action, especially within a current rule or the speaker's present decision, but it has no separate past or future inflected form.
past: had to / was allowed to; future: will have to; current rule: must + base verb
- 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.
Yesterday, every witness had to show photo identification.
Yesterday, every witness had to show photo identification.
Had to places an actual obligation in a completed past situation.
From January, suppliers will have to report emissions quarterly.
From January, suppliers will have to report emissions quarterly.
Will have to identifies a requirement that begins in a specified future period.
7. Forming modal constructions
After can, may, must and should, use the base form without to: may enter, must comply, should check. Form questions by inversion without do: May I enter? Ought requires to, while have to agrees with its subject: she has to.
Place not after the modal: must not disclose, may not enter, should not delay. Have to forms its negative with do: does not have to. Choosing a full or contracted form generally changes register and emphasis rather than the kind of deontic meaning.
subject + modal + (not) + base verb; subject + ought to / has to + base verb
- 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.
- modal —
- A modal verb expresses meanings such as ability, possibility or necessity: can, may, must.
May delegates bring an interpreter?
May delegates bring an interpreter?
May precedes the subject. The auxiliary do is unnecessary.
She has to renew the licence annually, but she needn’t apply in person.
She has to renew the licence annually, but she needn’t apply in person.
Has to agrees with she; needn't expresses the absence of a requirement.
8. Boundaries and use in real communication
Different modal meanings can coexist in one grammatical sentence. You mustn't be late, though I'm sure you won't be combines a prohibition with a prediction. The learning goal is to identify each part's function, not to forbid their combination.
Before writing your own statement, identify four things: who has authority, which action is regulated, whether it is required, permitted, forbidden or optional, and when the rule applies. Then choose a form suited to the genre: a conversational reply, official letter, policy or notice.
The handbook says we must encrypt the files; it does not prove that every file is already encrypted.
The handbook says we must encrypt the files; it does not prove that every file is already encrypted.
A deontic requirement concerns what ought to happen, not the truth of the current situation.
You may work remotely on Friday, but you must remain reachable during core hours.
You may work remotely on Friday, but you must remain reachable during core hours.
A single context naturally combines permission with a condition that must be met when using it.
Common mistakes
Incorrect:Employees must to display their badges.
Correct:Employees must display their badges.
After must, use the base form without to.
Incorrect:Do visitors may use the reading room?
Correct:May visitors use the reading room?
In a question, place modal may before the subject without adding do.
Incorrect:She have to renew the licence every year.
Correct:She has to renew the licence every year.
Have to agrees with its subject in the Present Simple: she has to.
Incorrect:Yesterday we must submit the signed original.
Correct:Yesterday we had to submit the signed original.
Use had to for an actual obligation in a completed past situation.
Incorrect:You don’t must attend the optional briefing.
Correct:You don’t have to attend the optional briefing.
Use don't have to to express no obligation. Don't must is not a grammatical construction.
Incorrect:The committee ought review the evidence again.
Correct:The committee ought to review the evidence again.
The semi-modal ought requires to before the base verb.