Have To / Must

Both must and have to express obligation, but they are not always used in the same way. Look at the examples:

I must call my grandmother today.
→ I feel it is important.
I have to wear a uniform at work.
→ It is a rule.
We must hurry or we'll miss the train.
→ I think it is necessary.
You have to show your passport at the airport.
→ The situation or the rules require it.

Must or have to?

→ Must usually expresses the speaker's opinion, feelings or strong recommendation.
→ Have to usually expresses an outside rule, law or situation.

You must see this film.
→ I strongly recommend it.
Students have to arrive before 8:00.
→ School rule.
I must remember to phone Mum.
→ My own obligation.

Tricky negatives

Must and have to have an opposite meaning when it comes to negatives, read the examples:

You mustn't park here. → It's forbidden, you cannot park here.
You don't have to park here. → It isn't necessary but you can

Have to for past and future

Must has no past or future form. Use have to / had to / will have to instead. Questions and negatives of have to use do/does/did.

I had to work yesterday.
We'll have to leave early.
Did you have to pay?
They didn't have to come.

Quick reference

must + verb → the speaker thinks it is necessary
have to + verb → obligation because of rules or circumstances
had to / will have to → past and future obligation
mustn't = forbidden
don't have to = not necessary