Could, May, Might

Could, may, and might all express possibility, but they do it in different ways. Could also covers past ability, polite requests, and suggestions. May doubles as a formal permission word. Might leans toward lower-probability guesses. Understanding where these three overlap and where they split is the key to using them naturally.

Could

Past ability

Could is the past form of can when it describes a general ability someone had before.

This only works for general ability over a period of time. For one specific occasion in the past, English uses was able to or managed to instead of could -- this is covered separately.

Present and future possibility

Could marks something as possible but not certain. It often suggests one option among several.

Polite requests

Could softens a request compared to can.

Suggestions

Could introduces an idea without pressure -- it presents one option among many.

May

Possibility

May signals that something is genuinely possible. In tone, it sits slightly higher on the formality scale than might.

Formal permission

May is the traditional verb for giving, asking, and refusing permission in formal contexts.

The negative may not is a formal refusal or prohibition.

There is no standard contraction of may not -- unlike can't or couldn't, the form mayn't is essentially extinct in modern English.

Might

Possibility (lower probability)

Might expresses a weaker or more uncertain possibility than may. In practice, the difference is often subtle, and many native speakers treat the two as interchangeable.

Past possibility in reported speech

When reporting what someone said, might serves as the past-tense form of may.

The negative is might not. The contraction mightn't exists but is rare outside British English.

May vs might for possibility

In everyday modern English, may and might are nearly interchangeable when talking about present or future possibility.

Both sentences mean the same thing. If there is any difference, may sounds slightly more formal and, for some speakers, implies a slightly higher probability. But this is a tendency, not a rule -- context and intonation matter more than the choice of modal.

One place where the distinction becomes clearer is past counterfactual situations -- events that did not actually happen. Here, might is the natural choice.

Using may have in such sentences is not strictly wrong, but might have sounds more natural to most speakers.

Could vs may/might for possibility

All three express possibility, but the flavour is different.

Could often suggests one option among several. It answers the question what are some things that could happen?

May / might focus on the genuine chance of something happening. They answer the question is it likely?

Compare these two:

The difference is subtle and context-dependent. In many everyday sentences, all three are interchangeable.

Negative forms

Couldn't is common in all registers. May not stays as two words -- the old form mayn't has disappeared from use. Mightn't appears occasionally in British speech but sounds unusual in American English.

Common mistakes

Could for past ability vs possibility

May for permission vs possibility

Negative forms

Modal + verb form

Past counterfactual