Unit 31.must and have to — grammar in use

A. We use must and have to to say that it is necessary to do something. Sometimes it doesn't matter which you use:
* Oh, it's later than I thought. I must go. or I have to go.
But there is a difference between must and have to 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's a really nice person. You must meet her. (= I say this is necessary)
* I haven't phoned Ann for ages. I must phone her tonight.
* I must get up early tomorrow. There are a lot of things I want to do.

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 the situation:
* You can't turn right here. You have to turn left. (because of the traffic system)
* My eyesight isn't very good. I have to wear glasses for reading.
* George can't come out with us this evening. He has to work.
* I have to get up early tomorrow. I'm going away and my train leaves at 7.30.
If you are not sure which to use, it is usually safer to use have to.
B. you can use must to talk about the present or future, but not the past:
* We must go now.
* We must go tomorrow. (but not 'We must go yesterday')
You can use have to in all forms. For example:
* I had to go to hospital. (past)
* Have you ever had to go to hospital? (present perfect)
* I might have to go to hospital. (infinitive after might)
In questions and negative sentences with have to, we normally use do/does/did:
* What do I have to do to get a driving licence? (not 'What have I to do?')
* Why did you have to go to hospital?
* Karen doesn't have to work on Saturdays.
C. Mustn't and don't have to are completely different:
You mustn't do something = it is necessary that you do not do it (so, don't do it):
* You must keep it a secret. You mustn't tell anyone. (= don't tell anyone)
* I promised I would be on time. I mustn't be late. (= I must be on time)
You don't have to do something = you don't need to do it (but you can if you want):
* You can tell me if you want but you don't have to tell me. (= you don't need to tell me)
* I'm not working tomorrow, so I don't have to get up early.
D. You can use 'have got to' instead of 'have to'. So you can say:
* I've got to work tomorrow. or have to work tomorrow.
* When has Ann got to go? or When does Ann have to go?