She Must Have Known — Modals of Deduction
B2 · Modals of Deduction
She Must Have Known
📖 From my experience:
My daughter was born in Réunion — by C-section, early morning. The doctor was calm. Too calm for what she was doing. Later I thought: she must have done this a thousand times. She can’t have been nervous. She might have been tired, though. All deduction. All from evidence. All in the past modal.
My daughter was born in Réunion — by C-section, early morning. The doctor was calm. Too calm for what she was doing. Later I thought: she must have done this a thousand times. She can’t have been nervous. She might have been tired, though. All deduction. All from evidence. All in the past modal.
🎯 Language focus:
Modals of deduction (past):
Modals of deduction (past):
must have + past participle = almost certain (positive)
can’t have + past participle = almost certain (negative)
might/could have + past participle = possible
She must have done this before. (I'm 95% sure)
He can't have left already. (Impossible — I just spoke to him)
They might have taken a different route. (It's possible)
✅ Context-based deduction:
Situation: The lights are off and the door is locked.
→ She must have gone home. (certain)
→ She can’t have forgotten — her keys are gone too. (impossible)
→ She might have left early. (possible)
Situation: The lights are off and the door is locked.
→ She must have gone home. (certain)
→ She can’t have forgotten — her keys are gone too. (impossible)
→ She might have left early. (possible)
Choose the right modal
1. The lights are on — he ___ still be home. (certain) must be (present deduction)
2. She passed without studying! She ___ been very talented. (certain past) must have been
3. He’s 90 — he ___ walked here. (impossible) can’t have walked
🧒 For younger learners: Show pictures with a mystery: ‘The ice cream is melted. What happened?’ Kids infer with ‘it must have been hot!’
👩🏫 Cambridge CELTA note: Modals of deduction are a key B2 (FCE) grammar point. Common error: ‘must have been’ confused with ‘should have been’ (advice). CCQ: ‘Are you sure? Almost sure → must have. Impossible → can’t have. Maybe → might have.’
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