1. MUST, HAVE TO and HAVE GOT TO: expressing the present.
Must, have to and have got to are all used to express obligation or the need to do something. They can be used interchangeably in the present tense, except that:
MUST suggests that it is the speaker who has decided that something is necessary (Internal Obligation)
HAVE TO and HAVE GOT TO suggest that somebody / something else has imposed the decision (External Obligation)
MUST : INTERNAL OBLIGATION
HAVE TO : EXTERNAL OBLIGATION
Have got to is characteristic of very informal speech. Have to sounds slightly more formal.
Compare the following:
I must clean the house before Sarah gets back.
I have to pay my income tax by Tuesday.
I've got to tidy up my room before the end of the afternoon.
You must come and visit us again soon. It's ages since we saw you.
With frequency adverbs such as always, often, sometimes, never, etc, have to is normally preferred:
I usually have to work on Saturdays so I hardly ever go away for the weekend.
2. MUST and HAVE TO: expressing the future and the past.
Must and have got to have no future or past tense forms. We cannot say: I had got to.../ I'll have got to.../ I'll must.../ I've must....
However we can also use must to express future as well as present intention, especially if it is the speaker who decides that something is necessary. But it cannot be used to express past intention. Have to is the only one of the three that possesses past and future forms.
Compare the following:
To get to Brighton by noon, I shall have to leave London before ten tomorrow.
To get to Brighton by noon, I must leave London before ten tomorrow.
You'll have to put the scaffolding up before you go on to the roof. It's not safe otherwise.
You'll have to have that tooth extracted as soon as you can. It's very badly infected.
We had to leave the party early. Billy was obviously exhausted.
We've had to cancel our holiday. Amanda is not fit enough for a hiking holiday this year.
3. MUST, HAVE TO and HAVE GOT TO in the interrogative.
Have to and have got to are often preferred in the interrogative, especially if the obligation is imposed from the outside.
Compare the following:
What time have you got to be back? By half past six.
How often do you have to travel to the UK on business? About once a month.
Must you leave right now? Are you sure you can't stay a little longer?
Do you have to leave now? I do, unfortunately. I've got so much work to do.
4. HAVE TO and MUSTN'T.
We have to use don't have to for the negative of must when there is no obligation or necessity to do something:
You don't have to eat at your desk. You can have a decent meal at our canteen.
I didn't have to work after all because my friend John was there.
You won't have to drive my sister to the airport next Monday. Julie's taking her.
We use mustn't to say that something is not allowed.
You mustn't drink if you're going to drive afterwards.
You mustn't drink that water. It's contaminated.
You mustn't lie under oath. If you do, that's perjury.
I mustn't forget my keys. I'll put them here so that I remember them.