Must

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.