"Have to" and "must" have
the same meaning in the affirmative and interrogative forms when referring
to obligation. Some grammarians think that "must" is slightly stronger,
but for all practical purposes, they mean the same thing:
Doctors have
to attend medical school for several years before
they can practice medicine.
Doctors must attend medical school for several years before
they can practice medicine.
While "have to" and "must" can be
used interchangeably, there are differences in usage, as Michael
Swan observes in Practical
English Usage (Oxford University Press, 1995):
Both verbs can
be used in British English to talk about obligation. (In American
English, have to is the normal form.) British
English often makes a distinction as follows. Must is used
mostly to talk about the feelings and wishes of the speaker and
hearer ?for
example, to give or ask for orders. Have (got) to is used
mostly to talk about obligations that come from "outside" ?for
example from laws, regulations, agreements and other people's orders.
Compare:
I must stop smoking. (I want to.)
I've got to [or I have to ?Rachel] 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 [or we have to
?Rachel] go home because of the baby-sitter.
?
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?)
The negative forms of "have to" and "must" carry very different
meanings.
The negative forms of "have to" ("don't have to," "doesn't have to," "didn't
have to")
mean that an obligation is not necessary:
We don't have to go
to work tomorrow; we can sleep until noon if we want!
Marcy doesn't have to take any more English courses; she has
satisfied the English requirement.
In times past, people didn't have to know how to use computers,
but now they do.
In contrast, the negative of "must" ("must not")
means that something is not permitted; it is prohibited.
In all cultures,
people must not kill or steal.
You must not tell
anyone about this. It is vital that the information be kept secret.
Children,
you must not run into the
street!
(To
see a related comment, "Had to/must in past tense of reported
speech," click here.)
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