Must Be, Might Be, Can't Be: Guessing in English
How English grades a guess — from almost certain to impossible — and the past forms that trip people up.
Somebody knocks at the door. You are not expecting anyone except your sister.
- That must be my sister. — I'm almost sure.
- That might be my sister. — possibly.
- That can't be my sister — she's in Rome. — impossible.
Same situation, three different levels of certainty, and the only thing that changes is the modal verb. This is not about permission or ability; it is about how confident you are.
The scale
| Modal | Confidence | Example |
|---|---|---|
| must | ~95% — the only explanation | You must be tired. |
| may / might / could | ~40% — one possibility | She might be at work. |
| can't | ~0% — impossible | It can't be true. |
May, might and could are close enough that you can treat them as interchangeable for guessing. Might is the most common in speech.
The two ends are not opposites of each other
Here is the trap. The opposite of must be is not mustn't be.
- He must be at home. (I'm sure he is.)
- He can't be at home. (I'm sure he isn't.)
Mustn't exists, but it means prohibition — you mustn't smoke here — and never "I'm sure it isn't". Reaching for mustn't to negate a deduction is one of the most persistent B1 errors.
Likewise the negative of might be is simply might not be: She might not be home yet.
Guessing about the past
Add have + past participle. The modal itself never changes form.
| Now | About the past |
|---|---|
| must be | must have been |
| might be | might have been |
| can't be | can't have been |
- The ground is wet. It must have rained.
- I don't know where she is — she might have missed the train.
- You can't have seen him yesterday; he was in hospital.
In speech have is reduced almost to nothing: must've, might've, can't've. It sounds like "must of", which is why that spelling is so common — and always wrong.
Never write "must of", "might of" or "could of". They exist only as a mishearing of the contraction. The word is always have.
Guessing about something in progress
Use be + -ing:
- She's not answering. She must be driving.
- He might be working late.
- They can't be sleeping — the lights are on.
And for the past: He must have been sleeping.
In conversation
Where is Tom?
A: Tom's not answering his phone.
B: He must be on the train — there's no signal in the tunnel.
A: He can't be. He said he was leaving at six.
B: Then he might have missed it. Or he could have forgotten to charge his phone.
Four guesses at four different strengths, and not one of them uses a word like maybe or probably. The modal does all of it.
Try it yourself
The quiz below focuses on the can't / mustn't split and on the past forms.