Difference between must and have to

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Both must and have to can be used to express the conclusion that something is certain. Note that have to is more common in American English.

Conclusions about the past are usually expressed with must followed by the perfect infinitive (have + past participle).

Necessity

Must and have to can both be used to talk about necessity. In American English, have to is more common.

Obligation

Both must and have to can be used to talk about obligation. In American English have to is the normal form. British English often makes a distinction between them. Must is used mostly to talk about the feelings and wishes of the speaker and hearer. 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:

Will have to is used to talk about future obligation, but have (got) to is preferred when arrangements for the future have already been made.

Had to is used to talk about past obligation. Must is used with the perfect infinitive (have + past participle) to express certainty about the past.

Must not and do not have to/ have not got to have quite different meanings. Must not is used to tell people not to do things; do not have to/ have not got to is used to say that there is no obligation.

Sections in this article

Modal Auxiliary Verbs
Can
May and Can: differences
Could
May
Might
Will
Would
Shall
Should
Should: other uses
Must
Must: uses
Must and have to: The Difference
Ought to
Need
Had better
Should, Ought and Must: The difference