Grammar › Modal Verbs › Deduction: must, might & can't
Deduction: must, might & can't
These modals show how certain you are about a present fact:
- must = you're almost sure it's true (He must be rich).
- might / may / could = it's possible (She might be busy).
- can't = you're almost sure it's false (That can't be true).
This is 'deduction', not obligation. For the past, add have + past participle (must have been, can't have known).
must, might/could, can’t for deduction
| Modal | How sure | Example |
|---|---|---|
| must | almost certain it’s true | He must be ill — he never misses work. |
| might / may / could | possible, not sure | She might be on her way. |
| can’t | almost certain it’s false | That can’t be the right address. |
Use modal + be or modal + base verb. This is about how sure you are, not about rules.
When to use each
1. must — you’re almost sure
- The lights are on, so someone must be home.
2. might / may / could — it’s possible
- I’m not sure where she is. She might be at lunch. It could be the traffic.
3. can’t — you’re almost sure it’s false
- He can’t be 40 — he looks so young. This can’t be true.
⚠️ For a negative deduction, use can’t (not mustn’t): She can’t know about it (I’m sure she doesn’t), not
she mustn’t know(which is a prohibition).
Common mistakes
1. ‘mustn’t’ for a negative deduction. That mustn’t be his car → That can’t be his car.
2. Adding ‘to’. She must to be tired → She must be tired.
3. Confusing deduction and obligation. You must be quiet (rule) vs You must be joking (deduction).
✏️ Practice: Deduction: must, might & can't
1. The lights are on and I can hear music. Someone ___ be home.
2. He's not answering. He ___ be asleep, but I'm not sure.
3. That ___ be Sara — she's abroad this week.
4. She's been working all night. She ___ be exhausted.
5. I'm not certain, but the keys ___ be in the car.
6. He earns very little, so he ___ be rich.
7. Choose the correct form: 'She ___ tired.'
8. Which modal means you're almost sure something is FALSE?
9. Choose the correct version of: 'That mustn't be the right answer.'
10. Deduction differs from obligation because it shows…
11. Fill in the gap
She's been awake for 30 hours. She ___ be exhausted. (almost sure)
Type one word.
12. Correct the mistake
That mustn't be the right address.
Type the whole corrected sentence.
13. Fill in the gap
I don't know where he is. He ___ be at the gym. (possible, not sure)
Type one word.
Frequently asked questions
How is 'must' for deduction different from 'must' for obligation?
Obligation: You must wear a seatbelt (a rule). Deduction: You must be exhausted (I'm sure you are). Same word, different job.
Which modal means 'it's impossible'?
'Can't': She can't be at work — it's Sunday. Don't use 'mustn't' for deductions.
Might, may or could — is there a difference?
For deduction they're almost the same: He might/may/could be asleep. All mean 'it's possible'.
How do I make a deduction about the past?
Use modal + have + past participle: She must have left; He can't have known; They might have forgotten.
👩🏫 For teachers
CEFR objectives for “Deduction: must, might & can't”:
- Can use must, might/may/could and can't for present deductions (CEFR B1).
- Can distinguish deduction from obligation.
- Can extend deductions to the past with modal + have + participle.
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