Page 156 - Basic English Usage

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We can also use have (got} to to talk about obligation. (See 156.) For the
difference between must and have (got) to, see 209.

Must is not used to talk about past obligation (must is used mainly for
giving orders, and you cannot give orders in the past). For the use of
had to, see 156.
For the difference between must not and don’t have to, haven't got to, don't need
to and needn't, see 209.

209 must and have to; mustn’t, haven’t got to, don’t
have to, don’t need to and needn’t

Must and have (got) to are not exactly the same. We usually use must
to give or ask for orders — the obligation comes from the person who is
speaking or listening.
We use have (got) to to talk about an obligation that comes from
‘outside’ — perhaps because of a law, or a rule, or an agreement, or
because some other person has given orders. Compare:
/ must stop smoking. (/ want to.)
!’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. | must make an appointment with the dentist.
/ can't come to work tomorrow morning because |’ve got to see the
dentist. (1 have an appointment.)
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 it a rule?)

Mustn’tis used to tell people not to do things: it expresses ‘negative
obligation’.
Haven't got to, don't have to, don't need to and needn't are all used to
say that something is unnecessary. They express absence of obligation:
no obligation. Compare:
You mustn’t tell George. (= Don't tell George.)
You don’t have to tell Alice. (= You can if you like, but it’s not
necessary.)
You don’t have to wear a tie to work. but you mustn’t wear jeans.
(= Wear a tie or not, as you like. But no jeans.)
Haven't got to, don't have to, needn't and don’t need to all mean more
or less the same.