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.
✏️ 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. Choose the correct version of: '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.
11. Fill in the gap
That ___ be his car. He doesn't have one. (impossible)
Type one word.
12. Correct the mistake
You can't smoke in the hospital. It's strictly forbidden. (stress the rule)
Type the whole corrected sentence.
13. Correct the mistake
You mustn't to be late.
Type the whole corrected sentence.
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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