Must vs Have To

have to is used to talk about our general responsibilities. Thus, to show that an obligation probably comes from some other person, not from us use have to. Examples: Every day I have to catch the train to work. We have to arrive at work before nine. I have to do a lot of work around the house. In all these cases the speaker receives an obligation from another person or simply explains that the choice does not necessarily depend on them.

have to behaves like a regular verb: I have to, she has to; Do I have to, Does she have to? I don’t have to, She doesn’t have to. I had to, She had to; Did you have to, Did she have to; I didn’t have to, She didn’t have to

use the full form of have: I have to and not I’ve to

must is used much less frequently than to have to. It is used when an authority gives a ‘subordinate’ an order (e.g. police officer to citizen, parent to child) - such situations are rare. It is also used to give a very strong recommendation or when there is a real emergency / urgency. Examples: You must go and see that movie – you would love it. Doctor, you must come immediately. I really must answer her email otherwise I might lose a great opportunity.