I have to do something = it is necessary to do it, I am obliged to do it:
- You can’t turn right here. You have to turn left.
- I have to wear glasses for reading.
- George can’t come out with us this evening. He has to work late.
- Last week Tina broke her arm and had to go to hospital.
- I haven’t had to go to the doctor for ages.
We use do/does/did in questions and negative sentences (for the present and past simple):
- What do I have to do to get a new driving licence? (not What have I to do?)
- Karen doesn’t have to work Saturdays, (not Karen hasn’t to)
- Why did you have to leave early?
You can use have to with will and might/may:
- If the pain gets worse, you’ll have to go to the doctor.
- I might have to work late tomorrow evening,
or I may have to work … (= it’s possible that I will have to)
Must is similar to have to:
- It’s later than I thought. I must go. or I have to go.
You can use must to give your own opinion
(for example, to say what you think is necessary,or to recommend someone to do something).
- I haven’t spoken to Sue for ages. I must phone her. (= I say this is necessary)
- Mark is a really nice person. You must meet him. (I recommend this)
We use have to (not must) to say what someone is obliged to do. The speaker is not giving his/her own opinion:
- I have to work from 8.30 to 5.30 every day. (a fact, not an opinion)
- Jane has to travel a lot for her work.
But must is often used in written rules and instructions:
- Applications for the job must be received by 18 May.
- (exam instruction) You must write your answers in ink.
You cannot use must to talk about the past:
- We had to leave early, (not we must)
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 don’t have to tell him, but you can if you want to.
- I don’t have to be at the meeting, but I think I’ll go anyway.
You can use have got to instead of have to. So you can say:
- I’ve got to work tomorrow.
I have to work tomorrow.
- When has Liz got to go?
When does Liz have to go?