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

Tense and time references with must

394a

Must

has no past form. 

Obligation

Obligation in the past is expressed by had to:

[someone talking about their childhood home]

We didn’t have water inside, we 

had to go to an outside pump to pump the

water to the house. 

(We didn’t have water inside, we must go to an outside pump …)

References to obligations which will or may occur at a given point in the future

are made with the future forms of have to rather than must:

Maybe one day he 

will have to accept the inevitable truth.

If he turns up after midnight, then he

’ll have to eat whatever he can find in 

the fridge.

Û

539 Glossary for any unfamiliar terms

Modality | 655

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Must

can be used to express the future-in-the-past, and occurs in reported clauses

in formal contexts:

She had little appetite for the food on the tray, but she knew that she 

must eat

something or else she would faint with hunger.

Deduction

The perfect form must have always refers to deduction, not obligation:

I 

must have been on the phone.

(it is likely/probable that I was on the phone)