Modals

We use must to indicate duty or obligation to do something; we could translate it as having to or duty. Be careful, because in its negative form, mustn't indicates prohibition. To talk about something you don't have to do, but you could do, we use not + have to: You don't have to eat that if you don't want to (You don't have to eat that if you don't want to), but You mustn' Don't smoke in here! (You must not smoke here! Do not smoke here!)

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.