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?