Modal Auxiliary Verb Must

Must is a modal auxiliary verb. It has no –s in the third person singular.

Must is followed by an infinitive without to.

Questions and negatives are made without do.

Must has no infinitive (to must) or participles (musting, musted). And it has no past tense. When necessary, we use other words, for example forms of have to.


Must indicates that it is necessary or very important that something happens. If you say that you must do something, you mean that you have a definite intention to do something in future.

If you tell someone else that they must do something, you are emphasizing that it is a good idea for them to do that.

Uses of must

To express a conclusion

Must can be used to express the conclusion that something is certain or highly possible.

Must in questions and negatives

Must is not often used to express certainty in questions and negative clauses. In questions we use can.

In negative clauses we generally use cannot/can’t to say that something is certainly not the case.

However, mustn’t is normal in question tags after must, and in negative questions.

To express necessity

Must is often used in affirmative sentences to say what is necessary, and to give strong advice and orders to ourselves and other people.

Must can be used in questions to ask about what the hearer thinks is necessary.

In American English, have to is more common.

Must not or mustn’t is used to say that things should not be done, or tell people not to do things.