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Modals
Quick rule
must = I'm sure it's true. might / may / could = it's possible. can't = I'm sure it's not true. Add 'have + participle' for the past.
The lights are off — they must have gone home.
It might just be a connection problem.
He can't have written this in an hour; it's forty pages.
She could be stuck in traffic.
Common mistake
✗ He mustn't have seen it. → ✓ He can't have seen it.
Short practice
Deduce: 'His car isn't here.' (sure he left) → He must have left.