Notes on modal forms expressing possibility

1 Should be and ought to be to express possibility In addition to the above examples, we can also express possibility with should be and ought to be: John should be/ought to beat home. John should be working/ought to be workingJohn should have left/ought to have leftby tomorrow etc. However, because should and ought to also express obligation [> 11.46] they can be ambiguous, so are not used as much as may/might/could to express possibility. For example, He should have arrived (ought to have arrived) yesterday could mean 'I think he probably has arrived' or 'He failed in his duty to arrive yesterday'.

2 Questions about possibility When we are asking about possibility, we may use Might ?, Could ? and sometimes Can ? and (rarely) May ?. (We do not normally use should and ought to in affirmative questions about possibility because of the risk of confusion with obligation): Might/Could/Can this be true? Might/Couldhe know the answer?

Might/Could/Can he still be working? (or be still working) Might/Couldhe be leaving soon? Might/Could/Canhe have been waiting long? Might/Couldhe have left by tomorrow?

Can is not always possible in questions like these, probably because of the risk of confusion with can = ability [> 11.10]. However, in questions like Can this be true?, can often indicates disbelief. Can is possible in some indirect questions: / wonder where he can have leftthe key?


11 Modal auxiliaries and related verbs

3 Negative questions about possibility Negative questions about possibility can be asked with Mightn't and Couldn't. May not (Not 'Mayn't*) can sometimes be used, as can Shouldn't and Oughtn't to: Mightn'the be at home now? etc Couldn't he know the answer? etc

4 Negative possibility Negative possibility is expressed with may not mightn't, can't and couldn't, but not usually with shouldn't and oughtn't to: He may not be(or have been) here etc. He may not be(or have been) working late etc. Can't + be often suggests disbelief: What you're saying can't betrue1 I can hardly believe it Can may be used in negative indirect questions: / don't think he can have lefthome yet or in semi-negatives' He can hardly beat home yet It's only 6