What's the difference between 'may', 'might', and 'could'?

All three express possibility, with subtle differences. 'May' suggests a slightly higher chance; 'might' is more tentative or hypothetical; 'could' often emphasises potential or capability. In practice, they're often interchangeable: 'It may/might/could rain later'. 'Might' is the most common in everyday speech.

How do I express past possibility?

Use modal + have + past participle: 'She might have left' (it's possible she left), 'He must have forgotten' (I'm almost sure he forgot), 'They could have called' (it was possible). The 'have' here is the auxiliary, not 'have' meaning 'possess'.

What does 'must' mean when expressing probability?

It means a strong logical conclusion based on evidence — not obligation. 'She must be tired' = I'm almost certain she's tired (because she's been working all day). The opposite is 'can't': 'She can't be tired' = I'm almost certain she's not tired. Both express deduction, not obligation.

Can I use 'must not' for negative probability?

Generally no — use 'can't' or 'couldn't' instead. 'He can't be home' (I'm sure he's not). 'Mustn't' usually means 'forbidden', not 'logically impossible'. The pair to remember: 'must' (sure positive) vs 'can't' (sure negative). 'Must not' for probability is rare and ambiguous.

How sure does 'might' make me sound?

About 30-50% sure. 'Might' suggests a real but limited possibility: 'It might rain' = maybe yes, maybe no. For higher certainty, use 'will probably' or 'should'. For lower certainty, use 'might just' or 'could possibly'. 'Might' sits in the middle of the probability scale.

Why is 'They might left' wrong?

After a modal, you need the base form of the verb. For past meaning, use 'have' + past participle: 'They might have left'. 'Might left' breaks the rule because 'left' isn't a base form. Always: modal + base verb (present) or modal + have + past participle (past).

Modal Verbs of Probability (may/might/could/must)

Expressing degrees of certainty: may, might, could, must

Last updated 13 June 2026

intermediate

Modal verbs express different degrees of certainty about the present, past, or future. 'May', 'might', and 'could' show possibility; 'must' shows strong logical conclusion; 'can't' shows strong negative conclusion. For past probability, use modal + have + past participle.

When to Use Modal Verbs of Probability (may/might/could/must)

•May/might/could for possibility: It might rain.
•Must for strong logical conclusion: She must be at work (I'm almost sure).
•Can't/cannot for strong negative conclusion: He can't be serious.
•Could have/may have/might have for past possibility: They might have left early.
•Must have for past deduction: She must have forgotten.

How to Form Modal Verbs of Probability (may/might/could/must)

Affirmative (+)

Subject + modal + base verb (She might come) / modal + have + V3 for past

Negative (-)

Modal + not + base verb (He must not be there) / modal + not + have + V3

Question (?)

Modal + subject + base verb? (Could it be true?)

💡 No 'to' after modals. Use have + V3 for past probability.

Examples of Modal Verbs of Probability (may/might/could/must)

✓She might join us.

✗She might to join us.

No 'to' after modals.

✓He must be tired.

✗He must to be tired.

Base verb after must.

✓They may have left early.

✗They may left early.

Past possibility uses have + V3.

✓He can't be serious.

✗He cannot to be serious.

No 'to' after cannot.

Common Mistakes with Modal Verbs of Probability (may/might/could/must)

💡 Practice Tips for Modal Verbs of Probability (may/might/could/must)

• Possibility: may/might/could; strong belief: must; strong negative: can't.
• Use have + past participle for past probability/deduction.
• No 'to' after modals; verb stays in base form.
• Avoid double modals (e.g., might can).

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When to Use Modal Verbs of Probability (may/might/could/must)

How to Form Modal Verbs of Probability (may/might/could/must)

Examples of Modal Verbs of Probability (may/might/could/must)

Common Mistakes with Modal Verbs of Probability (may/might/could/must)

✗She must to go.

✗They might left.

✗He can not comes.