Have to vs. must



Is there any difference between "have to" and "must" when talking about obligation ?

Posted 12 January 2002

"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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