Examples of modal forms for deduction

must and can't present reference

Certainty expressed by verb tenses:
He is here He lives here He is leaving

He isn't here He doesn't live here He isn't leaving

Deduction expressed by must be and can't be.
He must be here He must live here He must be leaving

He can't be here He can't live here He can't be leaving

perfect and past reference.
Certainty expressed by verb tenses
He was here He has left/He left early He has been/was

working late Deduction expressed by must have been and can't/couldn't have been He must have been here He must have left early He must have been

working late He can't have been here He can't have left early He can't have been

working late He couldn't have been here He couldn't have left early He couldn't have

been working late

Expressing deduction with 'must be' and 'can't be', etc.

The distinction between possibility (often based on speculation) and deduction (based on evidence) has already been drawn [> 11.27], The strongest and commonest forms to express deduction are must and can't. For teaching and learning purposes, it is necessary to establish the following clearly:

1 can't be (Not "mustn't be*) is the negative of must be.

2 can't have been (Not "mustn't have been*) is the negative of must have been.

Have to/have got to be (affirmative) can express, deduction in AmE: This has to be/has got to be the most stupid film I have ever seen

11 Modal auxiliaries and related verbs

Compare deduction [secondary use of modals > 11.3-4, 11.9] in:
He can't be thirsty He must be hungry

He can't have been thirsty He must have been hungry

with inescapable obligation [primary use of modals > 11.2, 11.4, 11.9] in:
He mustn't be careless He must be careful

He didn't have to be at the dentist's He had to be at the doctor s

We also use may'/might,'could and should/ought to for making deductions (as well as for expressing possibility); and, when we are almost certain of our evidence, we may use will and wont-That will be Roland I can hear him at the door That will have been Roland He said hed be back at 7 That won't be Roland. I'm not expecting him yet That won't have been Roland I'm not expecting him till 7

Again [> 11.31], it is possible to give varying responses to a question: Is Roland in his room?

- Yes, he is No, he isn't (certainty)

- Yes he must be. I heard him come in (deduction)

- No, he won't be He had to go out. (near-certainty)

- No, he can't be There's no light in his room (deduction)

Uses of modals for offers, requests, suggestions