What Are Modals of Deduction?
We use modals of deduction to say how sure we are that something is true right now, based on evidence. You aren't stating a fact — you're making a logical guess: "The lights are off, so they must be asleep."
Subject + modal (+ not) + base infinitive
The news must be fake. — you're almost certain it is fake
That story might / may / could be fake. — you aren't sure
This story can't be true. — you're almost certain it is not true
The verb after the modal is always the base infinitive — never to be, never is, never -ing.
The Certainty Scale
Choose the modal according to how confident you are.
| Modal | Certainty | Example |
|---|
| must | ~99% sure it's true | You've worked all day — you must be exhausted. |
| might / may / could | not sure — it's possible | Do you think that story might be fake? |
| can't | ~99% sure it's NOT true | He can't be at home — his car isn't there. |
She isn't answering her phone. She must be in a meeting.
→ Strong evidence, so you're almost certain.
This article has no named sources. It could be fake news.
→ It's possible, but you can't be sure.
That can't be Tom's car — he doesn't drive.
→ The evidence makes it impossible.
Negatives: can't vs might not
Be careful — the opposite of must (for deduction) is can't, not mustn't.
Impossible: can't + base infinitive
This story can't be true — certain details don't seem right.
Possibly not: might not / may not + base infinitive
This story might be true, but it may not be 100% accurate.
Common Mistakes
⚠ Watch out:
✗ The news must to be fake. → ✓ The news must be fake. (base infinitive, never to)
✗ He mustn't be at home — the lights are off. → ✓ He can't be at home. (use can't for impossible deductions)
✗ She must is very tired. → ✓ She must be very tired. (never conjugate the verb after a modal)