MUST - HAVE TO

Must and have to é usado para dizer que é necessário fazer algo. Algumas vezes não faz diferença qual deles usar.
■It is becoming later. I have to go. or I must go.

But there is a difference between have to and must and sometimes this is important:

Must is personal, we use must when we give our personal feelings:

You must do something = I (the speaker) say it is necessary;

■She is a really nice person. You must meet her.(=I say this is necessary)
■I have not called Mary for ages. I must call her tonight.

Have to is impersonal, we use have to for facts, not for our personal feelings:

You have to do something because of a rule or situation;

■You cannot turn left here. You have to turn right (because of the traffic system)
■Mike can't go to the movie with us tonight. He has to work.

Now, compare:

■I must get up early tomorrow. There are a lot of things I want to do.
■I have to get up early tomorrow. I am going away and my train leaves at 8:00.
Note: If you are not sure which one to use, it is safer to use have to.You can use must to talk about present or future, but not the past:

■We must go now.
■We must tomorrow.
You can use have to in all forms:

■In questions and negative sentences with have to, we use: do, does, did.