Cambridge Grammar of English Hardback with cd-rom a comprehensive Guide

Have (got) to and must

403e

Must

generally refers to obligations which originate from the speaker, and is often

used to express a sense of obligation or necessity, or a directive aimed at the

speaker or the listener. Have (got) to is generally more associated with external

obligations, originating from outside of the speaker:

I 

must buy a camera.

(directive to oneself)

I 

have to go back to the hospital for a scan next week.

(external obligation)

The negatives must not and don’t have to/haven’t got to are quite different. Must

not 

expresses prohibition or a negative directive; don’t have to/haven’t got to

expresses an absence of obligation:

You 

mustn’t tell anybody what I’ve just told you, okay?

I 

mustn’t forget to buy stamps when I’m in town.

You 

don’t have to have insurance, but you would be wise to buy some.

I 

haven’t got to go to Oxford next week. They’ve cancelled the meeting.

Û

394b for the negative of must

668 | Modality

Cambridge Grammar of English

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Don’t have to

can occasionally be used to reproach someone or to tell them not to

do something. In this usage, it is less direct than must not:

You 

don’t have to shout!

(Stop shouting.)

To make logical deductions, both must and have (got) to may be used (