Have to and must: obligation

Have to: present forms

Have to + base verb says an action is necessary. The base verb is the form you find in a dictionary, such as leave or bring. In the present simple, use have with I/you/we/they and has with he/she/it.

After does or doesn't, use have, not has. The final verb does not change: she has to work, not she has to works.

• My neighbour has to collect her son at four.
• Why do the guests have to leave so early?

Short answers use do or does: Does Eva have to attend? Yes, she does. / No, she doesn't. The negative answer means attendance is not necessary. It does not forbid her to attend.

Must: statements, negatives and questions

Must is a modal verb. It stays the same with every subject: I must, she must, they must. Do not add -s, to, or do/does/did.

• We must check the address before we leave.
• The cover must stay on the machine.
• You must not open this envelope.

Mustn't is the contracted form of must not. The full form is useful in formal rules or when you want to emphasise the instruction.

You can ask Must I sign both copies? or What must we bring? For an ordinary question about a requirement, you can use Do I have to sign both copies? Must-questions can sound formal and a little old-fashioned.

A positive short answer can be Yes, you must. For 'it is not necessary', answer No, you don't have to or No, you needn't. No, you mustn't means you are not allowed to do it.

Choosing have to or must

Both can express a strong need to do something. Have to can report a rule, a schedule or a situation. Must can present the speaker's own insistence, decision or instruction. This is a useful guide, not an absolute grammar test.

• I have to finish this drawing today. The customer needs it tomorrow.
• I must finish this drawing today. I have put it off long enough.

A person giving an instruction can use must even when it concerns an outside rule. A written rule can also use must. Do not reject must just because the sentence mentions a school or a law.

• Each visitor must sign the book at reception.
• We have to sign the book when we enter the building.

The first sentence states the requirement; the second reports it. Both are natural. In other contexts, either have to or must works with little difference. Treat the internal and external distinction as a guideline, not a rule that always holds.

These are grammar examples, not statements of a real organisation's current rules. US National Archives writing guidance uses must to impose a legal obligation and distinguishes it from will for a prediction. Must is not restricted to personal feelings.

Not necessary is not forbidden

This is the main negative contrast. Don't have to means you have a choice. Mustn't tells you not to do something. The same contrast applies to doesn't have to.

• Ada doesn't have to join the call. She can read the notes instead.
• Ada mustn't share the access code. It is private.

You can also say don't need to or needn't for no necessity. See Mustn't, don't have to and needn't. Can't can express a prohibition too, but it has other meanings such as inability.

There is a conversational extension: You don't have to interrupt every answer. This can be a criticism telling someone to stop, rather than a neutral offer of a choice. Cambridge records this less direct use. The basic no-necessity meaning is the place to start.

Past obligation: had to

Use had to for a requirement in the past. It is the same with every subject. Use have, not had, after did or didn't.

• Yesterday the lift broke, so we had to carry the boxes upstairs.
• Did Amir have to repeat the test?
• We didn't have to queue. Our tickets let us enter through a separate door.

Must has no ordinary past-tense form for this use. Must have + past participle is not the past of an obligation: She must have left is a conclusion about what happened. See Must and can't for deductions.

Didn't have to means there was no need. On its own it does not tell you whether the person did the action. I didn't have to cook, but I made some soup anyway is possible. See Mustn't, don't have to and needn't for the contrast with needn't have done.

Future requirements

Will have to expresses a future need, for example a new requirement or one that follows from a future situation. Use have after will with every subject.

• When this pass expires, I will have to apply for a new one.
• Will your team have to move next year?
• You won't have to carry cash. They accept cards.

You can also use present have to/has to or have got to for an already arranged future requirement.

• I have to meet the tutor at eleven tomorrow.

Must can refer to the future, even though it has no separate future-tense form. I must reply tomorrow is grammatical. Do not put will directly before must.

• We must remember to bring the keys next time.

This must expresses necessity, not a neutral prediction that something will happen. Use will for a prediction: The new office will be quieter. A statement with must be instead presents a conclusion from evidence.

Have got to: the informal alternative

Have got to has the same obligation meaning as have to and is more informal. Use have/has got to with a base verb. It can refer to now or a future requirement, but for past obligation and other tense forms use have to.

In she's got to, 's means has. In questions, put have/has before the subject; for a negative, put not after have/has. Do not combine do with have got to. Have to uses a different pattern: Do you have to work?

Have got to is not the ordinary present perfect meaning 'have obtained'. Here it expresses a present requirement: I've got to replace the battery means I need to do it. For I've had to replace the battery twice, see the next section.

More forms of have to

Unlike must, have to fits after another modal verb and in an infinitive or -ing form. The first have changes or follows the helper in the normal way.

In has had to, has is the perfect helper and had is the participle of have. This is different from must have done. Exact tense choice follows the usual rules for The present perfect: form and uses and The past perfect simple.

You can leave out a repeated verb when its meaning is clear: I don't want to move, but I have to. Keep to with have to. Must can stand without the repeated verb too: You must finish it. You really must.

Earlier completion as a requirement (advanced)

Have to have done can set earlier completion as a requirement. The first have belongs to have to; the second have begins the perfect infinitive, which looks back to an earlier action.

• To join the advanced group, you have to have completed the first course.

Completion of the first course is a condition for joining. Compare You have had to complete three forms today: that describes requirements during today. Do not confuse have to have completed with have had to complete.

You can also build a longer continuous pattern when the meaning needs it: Applicants had to have been working in the field for a year requires a period of work before the application. Do not read the perfect infinitive as a present perfect form of have to: the word order and meaning differ.

Strong suggestions, invitations and fixed expressions

Must can warmly encourage someone to do something rather than impose a rule. You have to / you've got to can also be an enthusiastic recommendation.

• You must come round when the garden is in flower.
• You've got to hear Lena's new song. I think you will love it.

Context matters: You must come tomorrow may be a friendly invitation or a firm requirement. For weaker advice, see Should, ought to and had better: advice and expectation.

I have to admit introduces an admission or emphasises what you are saying I must admit / you must admit can acknowledge a point, perhaps before a contrast I must say adds emphasis, often to a positive reaction

• I have to admit, the instructions confused me at first.
• You must admit the room is small, although the view is lovely.
• I must say, the new layout looks excellent.

A must-question can criticise an annoying action: Must you tap the table while I'm speaking? It asks about necessity in form, but conveys irritation in context. Context distinguishes the criticism from a genuine request for information.

Adverbs, reporting and formal extensions

An adverb such as also, always or never can follow must and come before the next verb: You must also include the date. Never already supplies a negative meaning: You must never reveal the answer is a prohibition.

In a past report, obligation must usually becomes had to: The trainer said we had to start at eight. Must can stay in a formal report of what was necessary from the past speaker's viewpoint: She knew she must continue. That is not an ordinary past-tense conjugation of must. See Reported statements: tense changes and say or tell.

In formal emphatic writing, a fronted negative phrase can invert must and the subject: Never must these records be altered. Compare the neutral order: These records must never be altered. See Inversion for emphasis for the wider pattern.

Have (got) to can also express a conclusion, not an obligation: There has got to be another explanation. Keep that meaning separate from requirements. See Must and can't for deductions for must and related conclusion patterns.

What comes next

Compare no necessity and prohibition in Mustn't, don't have to and needn't, advice in Should, ought to and had better: advice and expectation, and conclusions in Must and can't for deductions. Review related forms in Have and have got and Short answers, so do I, neither do I, and I think so.