Have to and must

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?