Modals on a scale of certainty

Degrees of certainty can be expressed on a scale: He is at home (= it's a certain fact, non-modal be) He could be at home (= doubtful possibility) He should be at home (= doubtful possibility) He ought to be at home (= doubtful possibility) He may be at home (= it's possible, but uncertain) He might be at home (= less certain than may)

He isn't at home (= it's a certain fact)

He can't be at home (= it's nearly certain)

He couldn't be at home (= more 'tentative' than can't)

He may not be at home (= possible, but uncertain)

He mightn't be at home (= less certain than may not)

(See 11.29ns1,3 for shouldn't and oughtn't to)

(See under deduction [> 11.32], for must be, can't be, etc.)

In speech, the element of doubt is increased with heavy stress: He could be at home (i e. but I very much doubt it).

Particular stress is also used in exclamations:

It 'can't be true¹ You 'can't 'mean it< You 'must be mistaken¹

Certain and uncertain responses to questions

Yes/No answers to questions can reflect varying degrees of certainty
felt by the speaker For example, a 'certain' question may elicit an
'uncertain' answer:
Does he like ice-cream? (direct question)

- Yes he does No, he doesn't ('certain' response)

- He might (do) He may (do) He could (do) (possibility)

- He mightn't He may not (uncertainty) Similarly, an 'uncertain' question may elicit a 'certain' answer:

Can he still be working? (disbelief)

Mightn't he be working? (possibility)

- Yes, he is No, he isn't ('certain' response)

Modals to express deduction

- He might (be) He may (be) (possibility)

- He may not be I don't think he can be (possibility)

- He can't be He couldnt be (disbelief) Of course, any other answer, not necessarily involving the use of a modal verb, may be available, depending on circumstances.

- / don't know I'm not sure I don't think so etc.

Be and have been are normally used in answers to questions with be Is he ill? - He may be Was he ill? - He may have been

Do often replaces other verbs:

Will you catch an early train? - I may do

Has he received my message? ~ He could have/could have done

Uses of modals to express deduction