Modals - 'must' and 'mustn't'

Modals – 'must' and '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.

How to use them

Must means 'Do it!' If you don't do it, you will be in trouble.

You must listen to the teacher.

Mustn't means 'Don't do it!' If you do it, you will be in trouble.

We mustn't forget our books.

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.