have (got) to and must

Where does the obligation come from?

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 – we haven’t seen your mother since Christmas.
  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 children.
• 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 I have to clean all the rooms? (or Must I …?)

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 here); but have (got) to is preferred when arrangements for the future have already been made. Compare:

• When you leave school you’ll have to find a job.
• I’ve got to go for a job interview tomorrow.

Going to have to is also possible.

• We’re going to have to repair that window.

Must can be used to give orders or instructions for the future.

• You can borrow my car, but you must bring it back before ten.

Will have to can be used to ‘distance’ the instructions (see here), making them sound less like direct orders from the speaker.

• You can borrow my car, but you’ll have to bring it back before ten.

Will need to can be used in the same way (see here).

note

For have to and have got to, (see here).

For the use of got with have, (see here).