English · B2 · grammar · 5 minutes
Modal deduction about the past
Grade confidence about past events with must, might, may, could, or can’t have + participle.
Understand the choice
These forms infer rather than report known history. Evidence supports a confidence level, and the modal communicates that level without converting it into certainty.
Core rule: Must have expresses a strong positive inference, might/may/could have a possibility, and can’t/couldn’t have a strong negative inference.
Notice it in context
They must have taken the wrong exit because they are an hour late.
Deben de haber tomado la salida equivocada porque llegan una hora tarde.
Must have taken is a confident inference about an earlier choice; because supplies present evidence.
She might have missed the message, so send it again.
Failure to see the message is one possible past explanation.
Might have keeps the explanation uncertain.
He can’t have walked here; his shoes are completely dry.
The dry shoes make walking here extremely unlikely.
Can’t have expresses strong negative deduction from evidence.
What to remember
- Must have is deduction, whereas had to describes past obligation.
- Could have may express possibility or an unreal unused opportunity; context distinguishes them.
Common trap: Do not use must not have when you mean a confident impossibility; can’t have is normally clearer.