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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, whereas have to and have got to suggest that somebody else has imposed the decision.

Have got to is characteristic of very informal speech. Have to sounds slightly more formal.

Compare the following:

• I must clean the house before mum gets back. I want her to find it all neat and tidy.
• Sorry, I can't come out now. I've got to tidy up my room before I'm allowed out.
• He has to attend the clinic every two weeks. He's really quite seriously ill.
• 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.
• They sometimes have to get their own suppers if their mother is working late.

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.

Compare the following:

• To get to Leeds by ten, I shall have to leave London before six tomorrow.
• To get to Leeds by ten, I must leave London before six 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. It's very badly infected.
• We had to leave the party early. Tom was obviously unwell.
• We've had to cancel our holiday. Tom is just not well enough for a walking holiday.

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? ~ Dinner's at seven. So by half past six really.
• How often do you have to travel to America on business?
  ~ About once every six months.
• Must you leave right now? Won't you stay a little longer?
• Do you have to leave now? ~ I do, unfortunately. I've got to collect my son from school.

have to and mustn't

We have to use have to for the negative of must when there is no obligation or necessity to do something:

• You don't have to drink champagne at the reception. You can have a soft drink.
• I didn't have to play after all. Jane turned up and could partner Alice.
• You won't have to drive Tom to the airport next Saturday. Julie's taking him.

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.