Must as an auxiliary verb

To express a conclusion

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

·         If A is bigger than B, and B is bigger than C, then A must be bigger than C.

·         There is the doorbell. That must be the postman.

Must in questions and negatives

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

·         Somebody is knocking at the door. Who can it be? (NOT Who must it be?)

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

·         It can't be your mother. She is in New York.

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

·         It must be nice to be a bird, mustn't it?

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.

·         We must get up early and start on our way.

·         We must build a strong army to defend the country.

·         I must stop smoking.

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

·         Must I go now?

In American English, have to is more common.

·         Do I have to go now?

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

·         You must not open this parcel until Christmas day.

·         You must not lie.