Modals of deduction: must have, can't have, might have
Certainty scales for present (must be / can't be) and past (must have been / can't have been) deductions
I can grade certainty about present and past situations with the modal deduction system.
Look at these examples. Can you spot the grammar pattern?
The lights are off — they must have gone to bed already.
The lights are off — they must have gone to bed already.
That can't be Tom's car — he sold his in March. It might be his brother's.
That can't be Tom's car — he sold his in March. It might be his brother's.
Pay attention to the highlighted parts. What do they have in common?
Modals grade your certainty:
| Certainty | Present | Past |
|---|---|---|
| sure it's true | must be | must have been |
| possible | may/might/could be | might have been |
| sure it's false | can't be | can't have been |
The negative of must (deduction) is can't, not mustn't: She can't be home — the lights are off. Past: He must have missed the bus.
- *She mustn't be home (deduction) → can't be
- *must had been → must have been
- Past deduction with simple modal: *He must missed → must have missed
Common Error Patterns
mustn't as deduction negative
Impossible → can't (have)
Simple modal for past deduction
modal + have + p.p.: must have missed
The lights are off — they must have gone to bed already.
The lights are off — they must have gone to bed already.
Past deduction from present evidence.
That can't be Tom's car — he sold his in March. It might be his brother's.
That can't be Tom's car — he sold his in March. It might be his brother's.
can't (impossible) + might (possible) on the scale.
Practice in course
Apply this grammar in B2 course exercises