* You use `have to’, `must’, and `mustn’t’ to talk about obligation and necessity in the present and future.
* You use `had to’ to talk about obligation and necessity in the past.
* You use the auxiliary `do’ with `have to’ to make questions.
* You use `have got to’ in informal English.
1 When you want to say that someone has an obligation to do something, or that it is necessary for them to do it, you use `must’ or `have to’.
You must come to the meeting tomorrow.
The plants must have plenty of sunshine.
I enjoy parties, unless I have to make a speech.
He has to travel to find work.
2 There is sometimes a difference between `must’ and `have to’. When you are stating your own opinion that something is an obligation or a necessity, you normally use `must’.
I must be very careful not to upset him.
We must eat before we go.
He must stop working so hard.
When you are giving information about what someone else considers to be an obligation or a necessity, you normally use `have to’.
They have to pay the bill by Thursday.
She has to go now.
Note that you normally use `have to’ for things that happen repeatedly, especially with adverbs of frequency such as `often’, `always’, and `regularly’.
I always have to do the shopping.
You often have to wait a long time for a bus.
3 You use `must not’ or `mustn’t’ to say that it is important that something is not done or does not happen.
You must not talk about politics.
They mustn’t find out that I came here.
Note that `must not’ does not mean the same as `not have to’. If you `must not’ do something, it is important that you do not do it.
If you `do not have to’ do something, it is not necessary for you to do it, but you can do it if you want.
WARNING: You only use `must’ for obligation and necessity in the present and the future. When you want to talk about obligation and necessity in the past, you use `had to’ rather than `must’.
She had to catch the six o’clock train.
I had to wear a suit.
4 You use `do’, `does’, or `did’ when you want to make a question using `have to’ and `not have to’.
How often do you have to buy petrol for the car?
Does he have to take so long to get ready?
What did you have to do?
Don’t you have to be there at one o’clock?
WARNING: You do not normally form questions like these by putting a form of `have’ before the subject. For example, you do not normally say `How often have you to buy petrol?’
5 In informal English, you can use `have got to’ instead of `have to’.
You’ve just got to make sure you tell him.
She’s got to see the doctor.
Have you got to go so soon?
WARNING: You normally use `had to’, not `had got to’, for the past.
He had to know.
I had to lend him some money.
6 You can only use `have to’, not `must’, if you are using another modal, or if you want to use an `-ing’ form, a past participle, or a `to’-infinitive.
They may have to be paid by cheque.
She grumbled a lot about having to stay abroad.
I would have had to go through London.
He doesn’t like to have to do the same job every day.