Modals of possibility (may, might, could)

Sometimes you're not sure — and English gives you three handy modals to say so: may, might, and could. They all express possibility, but they differ slightly in how certain or theoretical they sound.

It may rain later. (fairly possible — you're watching the clouds)
She might be at home. (possible but you're not sure)
That could be the answer. (a theoretical option worth considering)

These are modal verbs, so they follow the same easy rule you know from should and will: the modal never changes its form, and the verb after it is always the plain base form — no to, no -s, no -ed.

Quick form: subject + may / might / could + base verb → He may come. She might know. It could be true.

How to form them

The pattern is the same for all three modals, and the modal never changes — no matter the subject:

Subject	Modal	Base verb	Example
I	may	be	I may be* late.*
she	might	know	She might know* the answer.*
it	could	rain	It could rain* tomorrow.*
they	may	arrive	They may arrive* on Friday.*

Notice there is no third-person -s: ✅ she might know, never ❌ she might knows.

Degrees of certainty

The three modals occupy slightly different spots on the possibility scale, though may and might overlap a lot in everyday speech:

Modal	Feel	Example
may	fairly possible; a little more direct or formal	It may* be ready by noon.*
might	slightly more tentative; often interchangeable with may	I might* come to the party — I'm not sure yet.*
could	a theoretical option, one of several	The keys could* be in your bag — have you checked?*

In practice, may and might are frequently interchangeable — the difference is subtle, not absolute. Context matters more than a fixed certainty percentage. Could often suggests you're mentioning one of several possibilities, or inviting someone to consider an option they hadn't thought of.

In formal writing and news reporting, may is often preferred over might when the possibility feels real and current: The talks may lead to an agreement.

Present and future possibility

All three modals work for both the present moment and for future events — context makes the meaning clear:

Now:

She may be in a meeting. (right now, possibly)
He might have the information. (he possibly has it)
That could be the problem. (one possible explanation)

Future:

It might snow tonight.
They may arrive early.
We could win if we play well.

Talking about what might be happening now

When you want to say something is possibly in progress at this moment (or around now), use may / might / could + be + verb-ing:

She might be sleeping right now — it's probably better to call later.
They could be waiting for us outside.
He may be writing the report at the moment.

This mirrors the way you'd use be + -ing in other tenses to show an ongoing action.

Negatives

Add not after the modal:

Modal	Negative	Contracted	Example
may	may not	— (no common contraction)	It may not* work.*
might	might not	mightn't (mainly BrE, less common)	She might not* be home.*
could	could not	couldn't	That couldn't* be right.*

One important distinction: might not means possibly not (it's still possible that…), while couldn't in a possibility context often leans towards "that is not possible" or "I can't believe that is true":

She might not come. (possible that she won't come — but she still might)
That couldn't be true. (I can't see how that could be possible)

When you simply want to say it's possible that something won't happen, might not or may not is the usual choice.

May not has two meanings depending on context. With possibility: She may not be home = it's possible she isn't home. With rules and permissions: You may not enter = you are not allowed to enter. Context makes the meaning clear.

Questions

May and might are rarely used in questions about possibility — English prefers could or other structures:

Could this be a mistake?
Could she be at home?
Do you think it might rain?
Do you think she might be wrong?

Could question forms are natural in everyday speech; might and may questions sound quite formal and are less common.

This is about questions of possibility. May I…? is separate — it's one of the standard ways to ask for permission: May I leave early? For more on that use, see Requests & permission (can, could, may).

Common mistakes

❌ She might comes to the party. → ✅ She might come to the party. (base form after a modal — no -s)
❌ It may to rain later. → ✅ It may rain later. (no to after a modal)
❌ He might sleep right now. → ✅ He might be sleeping right now. (use might + be + -ing for an action possibly in progress at this moment)
❌ She couldn't come tomorrow (meaning: it's possible she won't come) → ✅ She might not come tomorrow. (couldn't suggests impossibility, not just possibility of not coming)
❌ They may are late. → ✅ They may be late. (modal + base form of be)

Quick check

Which modal or form fits each sentence?

I'm not sure where Ana is. She ____ be in the library. (might / may — both possible)
It ____ be cold tonight — a jacket might be a good idea. (could / might / should)
He ____ come to the meeting — he's very busy. (might not / couldn't)
Don't disturb him. He ____ on something important right now. (Add a modal + continuous form of work*)*

Show answers

She might be in the library. (also may — both fine; might sounds a touch more tentative)
It could be cold tonight. (also might — should here would mean expectation or advice, not possibility)
He might not come. (might not = possibly he won't; couldn't would suggest he is unable to come, which changes the meaning)
He might be working on something important right now. (or may be working / could be working)

Key takeaways

May, might, and could all express possibility. They are followed by the plain base form — no to, no -s.
May sounds a little more direct or formal; might sounds a little more tentative. In everyday English they are often interchangeable.
Could often presents one theoretical option among several possibilities.
For an action possibly in progress now: may / might / could + be + verb-ing → She might be sleeping.
Negatives: may not and might not both mean "possibly not"; couldn't in a possibility context leans towards "that can't be true."
Questions: could works most naturally → Could this be right? Might and may questions sound more formal.
May not has two distinct meanings: possibility (She may not be home) and prohibition (You may not enter) — context tells you which.
For logical deductions with must, can't, and might ("I'm almost certain she's at home"), see Modals of deduction: present.