Overview
Modals of deduction express how certain we are about something. 'Must' = strong certainty (He must be tired — I can see it). 'Can't/couldn't' = strong negative certainty. 'Might/may/could' = possibility. The structure for past deduction adds 'have + past participle': She must have left already.
Deduction Modals (Present)
- mustmust be — logically certain: He must be tired.
- can'tcan't be — logically impossible: It can't be right.
- mightmight/could be — possible: She might be at home.
Past Deduction
- must havemust have + pp — certain past: She must have left.
- can't havecan't have + pp — impossible past: He can't have said that.
- might havemight have + pp — possible past: They might have arrived.
Common Mistakes
- ❌ He must be left → ✅ He must have left (past deduction)
- ❌ She mustn't be home → ✅ She can't be home (impossibility)
- ❌ Must he be tired? → ✅ Could he be tired? (question form)
When to use
Drawing Conclusions
You must be exhausted after that long flight.
Expressing Doubt
She might have missed the train.
Ruling Things Out
He can't be the thief — he was with me.
Past Deduction
The door was open — someone must have broken in.