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

• He must go. (NOT He musts go…)

Must is followed by an infinitive without to.

• I must get some rest.
• You must finish the report today itself.
• You must not tell this secret to anyone else.
• Little children must not be left unattended.

Questions and negatives are made without do.

• Must we go now?
• You must not worry.

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.

• He will have to start coming on time. (NOT He will must …)
• We had to cancel the project. (NOT We musted …)

Meaning

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.

• I must get my hair cut.
• I must stop smoking.
• We must get someone to repair the roof.

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

You must stop lying.

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.