Possibility: May, Might & Could

When you are not sure whether something is true, happening, or will happen, English gives you a certainty scale — a set of modal verbs that express different levels of confidence. At one end, can't means "impossible"; at the other, must means "almost certain". In the middle sit the three modals of possibility: may, might, and could.

Beyond choosing the right level of certainty, you also need the right time frame. The same idea — "It is possible that..." — takes three different structures depending on when the situation happens:

This lesson covers all three time frames, from basic A2 possibility with may and might to B2 past speculation with may have, might have, and could have + past participle.

Note: "May" is also used for permission (May I sit here?), and "could" is also used for ability (I could swim when I was five) and polite requests (Could I borrow your pen?). In this lesson, we focus only on their possibility meaning. See Permission: Can, Could & May and Ability: Can, Could & Be Able To for their other uses.

May & Might: Present and Future Possibility

May and might are the two core modals for expressing possibility. Use them when you think something is possible but you are not sure.

Structure: Modal + Base Verb

Key point: Like all modals, may and might never change form — no -s, no -ing, no -ed:

• ✅ She might know the answer.
• ❌ She mights know.

Note: We rarely form direct questions with "may" or "might" for possibility. Instead, we use phrases like Do you think...? or Is it possible that...?:

• ❌ Might it rain? (grammatically possible but very rare)
• ✅ Do you think it might rain?

May vs Might: Is There a Real Difference?

In everyday English, may and might are almost interchangeable for expressing possibility. Both mean "it is possible that...":

• It may rain tomorrow. ≈ It might rain tomorrow.
• She may be at the library. ≈ She might be at the library.

There are only two small differences:

In practice, these differences are very subtle — native speakers use them interchangeably in most contexts. For your exercises, focus on the context clues ("I'm not sure", "nothing is certain", "let's wait and see") rather than trying to distinguish may from might.

Examples: Present Possibility

When you observe something and make a guess about the present:

• She isn't answering her phone. She may be busy.
• A: Where is David? B: I'm not sure. He might be at the library.
• Lisa looks tired. She may not feel well today.

Examples: Future Possibility

When you think something might happen but you are not certain:

• Take an umbrella. It might rain later.
• I'm not sure about tomorrow. I might go to the cinema or I might stay at home.
• We may move to a bigger house next year, but nothing is certain yet.

May Not vs Might Not

Both express a negative possibility — "it is possible that... not":

• Don't wait for Tom. He might not come to the party tonight.
• I may not be able to attend the wedding. I have to check my work schedule.
• The meeting might not start on time — I've heard the manager is stuck in traffic.

Important: "May not" and "might not" are never contracted in standard English:

• ❌ mayn't / ❌ mightn't (not used in modern English)
• ✅ may not / might not

👉 Practice May vs Might: Present and Future Possibility →

Could for Possibility — and the Certainty Scale

At B1 level, you learn that could is not just about ability or polite requests — it is also a modal of possibility, sitting right alongside may and might on the certainty scale.

Could + Base Verb for Possibility

When you are guessing or suggesting one possible explanation among several:

• A: Where is the cat? B: It could be hiding under the bed.
• That story could be true, but I find it hard to believe.
• Don't eat those mushrooms! They could be poisonous.
• This could be the right answer, but I'm not completely sure.

Could often implies "this is one possible explanation — there may be others". It is very similar to may and might for expressing possibility.

Could: Possibility or Ability? ⚠️

This is one of the most confusing points in English modal verbs. The word "could" has three very different meanings, and only context tells you which one applies:

The test: If you can replace "could" with "might" and the sentence still makes sense, it is about possibility:

• It could rain. → It might rain. ✅ (possibility)
• She could swim at five. → She might swim at five. ❌ (ability — not possibility)

For more on "could" for ability, see Ability: Can, Could & Be Able To.

The Full Certainty Scale

Now that you know may, might, and could for possibility, here is the complete certainty scale with must (strong deduction) and can't (impossibility):

Can't = the evidence makes it impossible:

• He speaks perfect French. He can't be from a different country — he's definitely French.

Must = the evidence makes it almost certain:

• She works twelve hours a day. She must be exhausted by the evening.

Note: The full deduction system with "must" and "can't" is covered in more depth in the Deduction & Speculation topic. This lesson focuses on the possibility zone (may/might/could), but you need to understand the whole scale to avoid choosing the wrong certainty level.

👉 Practice Could for Possibility: May, Might, Could & Can't →

Speculating About Right Now: Modal + Be + -ing

When you want to guess what is happening at this moment — an action in progress — use the continuous form: may/might/could + be + -ing.

Structure

Compare the simple and continuous forms:

Use the simple form for states (be busy, be tired, be at home) and the continuous form for actions you think are happening right now.

Examples

Guessing what someone is doing right now, based on clues:

• Tom isn't in his room. He might be playing in the garden.
• I can hear music from next door. The neighbours could be having a party.
• She's been in the meeting room for hours. She may be planning a new project.
• I can smell something burning. Dad could be cooking dinner.
• It's very quiet upstairs. The children might be sleeping already.

Negative Word Order ⚠️

This is a common error zone. The word "not" goes after the modal verb, before "be":

The rule: modal + NOT + be + -ing. Never put "not" after "be", and never drop "be".

Common Structure Mistakes

Watch out for these incorrect forms that appear frequently:

👉 Practice Speculation About the Present: Modal + Be + -ing →

Past Speculation: Modal + Have + Past Participle

When you want to guess about something that possibly happened in the past, use: may/might/could + have + past participle.

Structure

Examples: Guessing About the Past

When you see evidence now and guess what possibly happened before:

• The ground is wet. It may have rained last night.
• I can't find my wallet. I could have left it at the restaurant.
• She didn't come to the party. She might not have known about it.
• He failed the test. He might not have studied enough.
• The plants are all dead. The cleaner might have forgotten to water them.

Negation Structure ⚠️

Just like the continuous form, "not" goes after the modal, before "have":

Critical rule: Never use didn't after a modal verb. The past meaning comes from "have + past participle", not from "did":

• ❌ She might didn't know.
• ✅ She might not have known.

The Past Certainty Scale: Couldn't Have & Must Have

The certainty scale also works in the past tense:

Couldn't have vs might not have — the difference matters:

• She couldn't have cheated. = It was impossible — I am certain she did not cheat.
• She might not have cheated. = It is possible she did not cheat — I am not sure.

Can't have is also used for impossible past, with the same meaning as "couldn't have":

• He can't have run the whole marathon. He was still full of energy at the finish line.

Past Continuous Speculation

For guessing about a past ongoing action, combine both structures: modal + have + been + -ing:

• He said he was at work, but he may not have been telling the truth.
• She could have been sleeping when you called.

This form is less common but appears at B2 level and above.

👉 Practice Past Speculation: May Have, Might Have & Could Have →

Common Mistakes to Avoid

Quick Summary

The Certainty Scale

Decision Guide: How to Choose

Step 1 — How certain are you?

Step 2 — What time frame?

Step 3 — Negative? Where does "not" go?

Always after the modal, before everything else:

• might NOT be
• might NOT be doing
• might NOT have done

Practice Tips

1. Use the certainty scale: When you want to guess about something, first decide how certain you feel — impossible (can't), possible (may/might/could), or almost certain (must)? This one decision determines which modal to use.
2. Time frame check: After choosing your modal, ask: am I talking about now (base verb), an action in progress (be + -ing), or the past (have + past participle)?
3. Listen for clues in conversation: Phrases like "I'm not sure", "I think", "maybe", and "perhaps" signal that the speaker is using possibility modals. Train your ear to notice them.
4. Practice past speculation with news stories: Read a news headline and speculate about what might have happened: "The shop is closed early — they could have closed for a holiday."
5. Remember the "not" position: The most common structural mistake is putting "not" in the wrong place. Always: modal + NOT + be/have. Say it out loud until it feels natural.

Practice All Exercises

Ready to practise expressing possibility and probability with modal verbs? These online multiple choice exercises cover may, might, and could for possibility — from A2 basics to B2 past speculation, all with answers and explanations. Start with Set 1 for the fundamentals, or jump to Set 4 for the advanced challenge: