Must and have to is used to say that it is necessary to do something. Sometimes is does not make difference which you use:
- 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.
- 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:
- I had to go to school.(past)
- Have you had to go to school.(present perfect)