When you look at the evidence and draw a conclusion, you're making a deduction. In this lesson, we focus on three modals that are especially useful for present deductions: must, can't, and might.
- She hasn't slept in two days. She must be exhausted. (almost certain it's true)
- He says he passed the test without studying. He can't be telling the truth. (almost certain it's NOT true)
- The lights are off — she might be asleep already. (possible, but you're not sure)
These aren't guesses pulled from nowhere; they're logical conclusions from the clues in front of you.
The key pattern: subject + must / can't / might + base form → It must be cold. She can't know. He might be home.
The three modals at a glance
| Modal | What it signals | Example |
|---|---|---|
| must | near-certainty that something IS true | She must know — she's been working here for years. |
| can't | near-certainty that something is NOT true | That can't be right — I checked the figures myself. |
| might | open possibility — you genuinely don't know | They might be stuck in traffic. |
Like all modals, these are followed by the plain base form — no to, no -s, no -ing directly after the modal itself.
Must: drawing a positive conclusion
Use must when the evidence points so strongly in one direction that you're almost sure something is true. You're not stating a fact — you're reasoning to a conclusion.
- You've been on your feet all day. You must be tired. (strong evidence → confident deduction)
- She speaks six languages. She must enjoy learning languages.
- He looks just like Tom. They must be related.
The conclusion is based on what you can see, know, or infer — not on wishful thinking.
It's easy to mix up deduction must with obligation must ("You must hand in your assignment on time"). Context makes it clear: deduction must is about what you conclude from evidence, not about rules or requirements. For obligation, see Have to / must (obligation).
Can't: drawing a negative conclusion
Use can't when the evidence makes something feel impossible or almost certainly untrue — it's the negative mirror of must.
- The museum closes at 5, and it's already 6. It can't be open. (very unlikely to be true)
- This can't be the right address — there's nothing here.
- He can't know the answer — it was announced after he left.
Important: when making a negative deduction, use can't, not mustn't. ✅ It can't be true. — ❌ It mustn't be true. In standard learner English, mustn't signals prohibition ("you are not allowed to"). Can't is the deduction form.
Might: an open possibility
Use might (or could) when you're genuinely uncertain — the evidence doesn't point strongly either way. You're considering a possibility, not committing to a conclusion.
- She's not answering. She might be in a meeting.
- It might mean something different in that context — I'm not sure.
- He might know the answer — it's worth asking.
Might is weaker than must. If you said She must be in a meeting, you'd sound more confident that it's really happening. With might, you're leaving the door open.
For more on using may, might, and could to express possibility (rather than deduction from evidence), see Modals of possibility (may, might, could).
Deduction about what's happening right now
When you want to make a deduction about an action in progress at this moment, use must / can't / might + be + verb-ing:
- The music is so loud — they must be having a party.
- He's only wearing a T-shirt. He can't be feeling very warm.
- She's not at her desk. She might be having lunch.
This continuous form says: "Based on the evidence, this is (or isn't, or possibly is) happening right now."
| Deduction | Form | Example |
|---|---|---|
| almost certainly happening | must be + -ing | They must be waiting for us. |
| almost certainly NOT happening | can't be + -ing | She can't be sleeping — the lights are on. |
| possibly happening | might be + -ing | He might be working late — no one can reach him. |
Certainty scale: deduction vs possibility
It's useful to see where deduction modals sit relative to certainty:
| Certainty | Modal | Example |
|---|---|---|
| Almost certain (positive) | must | She must know — she's the expert. |
| Open possibility | might / could | She might know — it's worth asking. |
| Almost certain (negative) | can't | She can't know — she just arrived. |
Must and can't both show high confidence — must for a positive conclusion, can't for a negative one. Might sits in the uncertain middle, where the evidence doesn't decide the question.
Common mistakes
- ❌ It mustn't be true — the numbers don't add up. → ✅ It can't be true. (mustn't = not allowed; for negative deduction use can't)
- ❌ She must knows the answer. → ✅ She must know the answer. (base form after a modal — no -s)
- ❌ He must be know the answer. → ✅ He must know the answer. (no bare be before a plain verb; use must know for a state, must be knowing is not natural)
- ❌ They must work in there right now. → ✅ They must be working in there right now. (for a deduction about an action in progress, use must be + -ing)
- ❌ She can't to be serious. → ✅ She can't be serious. (no to after a modal)
Quick check
Choose the best modal for each deduction.
- The exam was three hours long, she finished in forty minutes, and she got every question right. She ____ be very good at maths. (must / can't / might)
- I can hear voices inside. Someone ____ be home. (must / can't)
- His cash is always in his wallet, and his wallet is at home. He ____ be carrying any cash. (must / can't)
- She's wearing headphones, but the screen is turned away from us. She ____ be listening to music. (must / might)
Show answers
- She must be very good at maths. (the evidence — speed and accuracy — points clearly in that direction)
- Someone must be home. (the voices are strong, direct evidence)
- He can't be carrying any cash. (his cash is in his wallet and his wallet isn't with him — near-impossible)
- She might be listening to music. (she could be, but we can't see the screen to confirm)
Key takeaways
- Must = almost certain the conclusion IS true — the evidence strongly supports it.
- Can't = almost certain the conclusion is NOT true — it would contradict what you know.
- Might (or could) = possible but uncertain — the evidence doesn't decide it either way.
- All three are followed by the plain base form: no to, no -s, no -ing directly after the modal.
- For a deduction about an action in progress now, add be + -ing: must be working, can't be sleeping, might be waiting.
- Can't is the negative deduction form — not mustn't (which means "not allowed to").
- Making a deduction about the past uses a different structure: must have / can't have + past participle — that's covered at Modals of deduction: past.