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

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397 Need and 403d Have to, have got to

Deduction

The negative of must when denying or negating logical deductions or conclusions

is can’t/cannot:

[speakers are measuring distances on a map]

A: There’s no way that’s thirty miles. 

B: That can’t be right. No it isn’t. 

A: Maybe the scale’s wrong. 

B: Mm, must be. 

656 | Modality

Cambridge Grammar of English

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Mustn’t

is also used to express negative deductions, especially in more informal

spoken contexts:

[talking about time sheets to record work done]

But she said ‘Oh you 

mustn’t have filled it in because I haven’t got any record

of you working those two shifts.’ But I’m sure I did. I’m sure I filled it in but

now I can’t remember. 

(or: You can’t have filled it in because …)