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

Negative of must

394b

✪

Obligation: must not and need not

Must not

is used to forbid something. Need not or the negative forms of 

have (got) to

or the negative of the lexical verb need to are used to express 

an absence of obligation:

[parent to child]

A: Samantha? 

B: What?

A: You must not leave stuff like this under the sofa! 

(forbidding something)

[pharmacist advising a customer about a medical product]

This 

needn’t be kept in the fridge.

(there is no obligation or necessity to keep this in the fridge; ‘This must not

be kept in the fridge’ would mean it is forbidden/wrong/dangerous to keep it

in the fridge)

[speaker B is a pensioner who enjoys free bus travel]

A: The bus is going to be about £1.

B: Well I don’t have to pay.

(Well I mustn’t pay.)

We should have a general discussion at this stage. Er, we 

don’t need to reach

any final decisions.