Obligation, prohibition

Have to/must + V1

You have to wear a seatbelt.
We must be on time for the test today.

Past and future forms of have to:

had to, will have to

I had to pay a fine for exceeding the speed limit.

All the employees will have to attend a 2-week management course.

Past form:

Note:

a) There is a slight difference in the way must and have to are used.

Have to is used to express general, external obligations coming from someone, e.g. rules and laws:

We have to wear a uniform at work.

Must is used for specific (i.e. on one occasion) or personal obligations coming from the speaker:

I must finish the project tomorrow.

We must help our parents.

b) Have to is more common in questions than must:

Why do we always have to work overtime?

c) Have got to can be used instead of have to in colloquial English:

I’ve got to call my mom now.

can’t/mustn’t/be not allowed to + V1

You mustn’t use your phone until you get off the plane.

You can’t smoke here.

You’re not allowed to park here.

Don’t/doesn’t have to/need to

You don’t have to pay for the exhibition – it’s free.

We don’t need to hurry, we have plenty of time.

Past form:

Didn’t have to/need to

You didn’t have to phone there yesterday.