MUST - HAVE TO

We can use 'must' to show that we are certain something is true. We are making a logical deduction based upon some clear evidence or reason.

• There's no heating on. You must be freezing.
• You must be worried that she is so late coming home.
• I can't remember what I did with it. I must be getting old.
• It must be nice to live in Florida.

We also use 'must' to express a strong obligation. When we use 'must' this usually means that some personal circumstance makes the obligation necessary (and the speaker almost certainly agrees with the obligation.)

• I must go to bed earlier.
• They must do something about it.
• You must come and see us some time.
• I must say, I don't think you were very nice to him.

We can also use 'have to' to express a strong obligation. When we use 'have to' this usually means that some external circumstance makes the obligation necessary.

• I have to arrive at work at 9 sharp. My boss is very strict.
• We have to give him our answer today or lose out on the contract.
• You have to pass your exams or the university will not accept you.
• I have to send a report to Head Office every week.

We use 'don't have to' (or 'haven't got to' in British English) to state that there is NO obligation or necessity.

• We don't have to get there on time. The boss is away today.
• I don't have to listen to this. I'm leaving.
• You don't have to come if you don't want to.
• He doesn't have to sign anything if he doesn't want to at this stage.
• I haven't got to go. Only if I want to