Can, Could, May, Might: Talking About Possibility

When you're not certain about something, English uses modal verbs to show how sure you are. Might, may and could all mean "possibly" — a real but uncertain chance. The good news: they barely change form, and they're followed by the base verb every time. If you are sure, use will. This guide focuses on uncertain possibility.

The basic idea

For present or future possibility, may, might and could are largely interchangeable. All take the base verb. They are close, but not identical: may is a little more formal, might can sound slightly less likely, and could often presents one possible outcome.

To mean "maybe not", use may not or might not. Don't use couldn't for this meaning. Can't / couldn't usually means inability, impossibility, or "I'm sure it isn't true."

Can vs could for possibility

Use can for general possibility (things that are possible in principle), but could / might for a specific future chance.

In standard learner English, "It can rain tomorrow" usually sounds unnatural if you mean one uncertain future event. For a specific future possibility, use could / may / might: "It might rain tomorrow." Use can for general possibility or ability: "It can rain a lot here in spring" / "We can still win."

Talking about the past

For past possibility, use may / might / could + have + past participle.

Common mistakes

Adding -s or "to" after a modal. Modals never change and take the base verb.

"Can" for a specific future possibility.

"Couldn't" for negative possibility. To say "maybe not", use may not / might not. Can't / couldn't usually means "impossible", "not able to", or "I'm sure it isn't true."

Quick exercise

Choose a natural modal of possibility.

1. Take an umbrella — it ___ rain later.
2. Deserts ___ be extremely hot during the day. (general)
3. I can't find my keys. I ___ have left them at work.
4. She ___ not come tonight; she's feeling ill.
5. The meeting ___ be cancelled; we're waiting for confirmation.