Must or have to

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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