Must / Mustn’t

We can use must and mustn’t to talk about rules and obligations.

I must do my homework. We must wear our school uniform.
You mustn’t be late for class. We mustn’t speak when the teacher’s speaking.

Must means ‘Do it!’ If you don’t do it, you will be in trouble. Use must to talk about what it is necessary to do.

You must listen to the teacher.

Mustn’t means ‘Don’t do it!’ If you do it, you will be in trouble. Use mustn’t to tell someone not to do something

We mustn’t forget our books.

Rules :

Use the infinitive without to after must and mustn’t.

Children must respect their parents.
Parents must take care of their children.
You mustn’t shout in the library.

We do not add an –s to the third person with must.
We say :  he must do his homework (not : he musts do his homework).

MUST or MUSTN’T ?