These modal verbs are used to express obligation

e.g.

I have to go to school

I must go to school

I have to do my homework

I must do my homework

But be careful with the negative form!

Don’t have to – Mustn’t

Don’t have to: is used to say that something is not necessary

Mustn’t: is used to say that something is forbidden

e.g.

We are students, so we don’t have to pay to get in!

You mustn’t drink and drive!