Possibility: may, might, could
Weaker conviction regarding the truth of an assertion is expressed as the epistemic possibility of an event occurring or being true. English speakers make use of the modal auxiliaries may, might and could, all glossed as ‘it is possible that . . . ’.
This may be a dinosaur’s footprint.
This might be a dinosaur’s footprint.
This could be a dinosaur’s footprint.
Might and could, though historically past forms, do not in such cases refer to past time, but to a present state of affairs. Past time is expressed by have + en.
This may/might/could have been a dinosaur’s footprint.
Can is not used in positive declarative clauses that express epistemic possibility. Instead, could has been taken up for this purpose. Can’t replaces must, however, in the negation of possibility (= it is not possible that). (The modal adverb possibly is typically added for emphasis):
It can’t possibly be a dinosaur’s footprint.
It is not easy to claim with certainty that may, might and could represent points on a scale of confidence or, in other words, that one or other expresses either a stronger or a remoter possibility. In the positive examples, the three modals are interchangeable, with little difference to the message. They may all occur in one utterance, as in the following:
I may be a few minutes late; it might be seven o’clock before I can get away; it could even be half-past.
They can all be intensified by (very) well, which heightens the possibility, and by just about, which lowers it:
It could very well /just about be a dinosaur’s footprint.
We must consider where we’ve been before in this House, that our intelligence as it stands might just be wrong because it was before and we’ve got to be very very hard in testing it . . . (David Davies in the House of Commons).
The use of these modals in the press seems to suggest may and might as tentative, with could expressing a stronger possibility, as shown in these extracts from The Guardian:
The change in Britain’s ethnic make-up may already be enough to cost (the Government) the next election.
The ethnic minority vote could be a key factor in the next elections.
(with regard to fracking) In Texas alone, about 30 communities could run out of water by the end of the year.
