Grammar › Modal Verbs › Can't vs Mustn't
Can't vs Mustn't
'Can't' and 'mustn't' are easy to confuse. They mean different things:
- Can't — impossible, or unable (I can't lift it; that can't be true).
- Mustn't — forbidden; a rule telling you not to (You mustn't run by the pool).
For deductions ('it's impossible that…'), use 'can't', never 'mustn't': He can't have finished already.
The difference in one table
| Can’t | Mustn’t | |
|---|---|---|
| Meaning | impossible / unable / not allowed | forbidden (a strong rule) |
| Used for guesses? | yes (negative deduction) | no |
| Example | It can’t be true. | You mustn’t be late. |
Can’t is about what’s impossible; mustn’t is about what’s forbidden.
When to use each
1. Can’t — impossible or unable
- I can’t lift this box; it’s too heavy. (unable)
- That can’t be right — check it again. (impossible)
2. Mustn’t — forbidden
- You mustn’t use your phone during the exam. (a rule)
- We mustn’t forget to lock up. (strong ‘don’t’)
3. Only ‘can’t’ for negative deductions
- She can’t have left — her coat is still here. (it’s impossible)
⚠️ Typical error: don’t use mustn’t for a guess. He can’t be at work — it’s Sunday — not
he mustn’t be at work.
Common mistakes
1. Using ‘mustn’t’ for a deduction. That mustn’t be his car → That can’t be his car.
2. Using ‘can’t’ when you mean forbidden and stated as a rule. Both are common, but for a firm instruction, mustn’t is clearer: You mustn’t tell anyone.
3. Adding ‘to’. You can’t to go / You mustn’t to go → You can’t go / You mustn’t go. Pro tip: impossible → can’t; forbidden → mustn’t; guessing ‘no way’ → can’t.
✏️ Practice: Can't vs Mustn't
1. This box is far too heavy — I ___ lift it.
2. During the test, you ___ look at your neighbour's answers.
3. That ___ be Sara — she's in Australia right now.
4. You ___ touch the paintings in the museum.
5. He ___ have finished the whole book already — he only started today!
6. I'm sorry, I ___ come to your party — I'll be away.
7. We ___ forget to send the invoice, or we won't get paid.
8. Correct this: 'That mustn't be the right answer.'
9. Which sentence expresses a PROHIBITION?
10. The door's locked from inside, so she ___ have gone out this way.
Frequently asked questions
What is the difference between 'can't' and 'mustn't'?
'Can't' means impossible or unable (I can't swim; that can't be right). 'Mustn't' means forbidden (You mustn't touch that).
Which one do I use for a rule?
Both can express rules, but 'mustn't' stresses prohibition (You mustn't cheat), while 'can't' often reports what's not allowed (You can't park here).
Can 'mustn't' be used for guessing?
No. For a negative deduction, use 'can't': She can't be forty — she looks so young. 'Mustn't' is only for prohibition.
👩🏫 For teachers
CEFR objectives for “Can't vs Mustn't”:
- Can distinguish 'can't' (impossibility/inability) from 'mustn't' (prohibition) (CEFR B1).
- Can use 'can't' for negative deductions.
- Can avoid using 'mustn't' for guesses.
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