Grammar: Have to and Must

Read the rules:

Have to

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.

·        I may have to work… (= it's possible that I will have to)

Must

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). Have to is also possible:

·        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.