Page 368 - Practical English Usage 3ed - Michael Swan, Oxford

6 indirect speech
Must can be used after a past reporting verb as if it were a past tense. The doctor said that I must stop smoking.
Obligation can also be reported with had to and would have to. The doctor said that I had to I would have to stop smoking.
7 must and should
Should can be used as a weaker form of must (see 520). Compare: That carpet must be cleaned. (= It is absolutely necessary.)
That carpet should be cleaned. (= It would be a good idea.)
361 must (4): advanced points
1 must and have to
In statements about obligation with must the obligation most often comes from the speaker (and in questions, from the hearer). To talk about an obligation that comes from 'outside' (for instance a regulation, or an order from somebody else), we usually prefer have to. Compare:
- I must do some more work; I want to pass my exam.
In my job I have to work from nine to five. (More natural than ... I must
work from nine to five.)
- We must go to New York soon and see your mother.
My wife'S an interpreter: she often has to go to New York. (More natural than ... she must often go to New York.)
- I must stop smoking. (I want to.)
I've got to stop smoking. (Doctor's orders.)
- This is a terrible party. We really must go home.
This is a lovely party, but we've got to go home because of the baby-sitter.
- I've got bad toothache. I must make an appointment with the dentist.
I can't come to work tomorrow morning because I've got to see the dentist.
- You really must go to church next Sunday - you haven't been for ages. (I am telling you to.)
Catholics have to go to church on Sundays. (Their religion tells them to.)
- Must you wear dirty old jeans all the time? (Is it personally important
for you?)
Do you have to wear a tie at work? (Is there a regulation?)
Have to can also be used to talk about obligation coming from the speaker or hearer, in the same way as must. This is normal in American English (which uses must less often in this sense), and is becoming very common in British English.
I really have to stop smoking. (OR I really must ...) Do [ have to clean all the rooms? (OR Must I ...n
For have 10 and have gol to, see 239.
2 future obligation: will have to. have (got) to and must
Will have to is used to talk about future obligation (will must is impossible - see 358); but have (got) to is preferred when arrangements for the future have already been made. Compare:
must (4): advanced points 361
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