Level: Intermediate-Advanced
Modal verbs for deduction are used when trying to see whether something is true or not. The way we choose these modal verbs is an indication of how big we think the possibility is. Some modals of deductions about the past are must have, might have/may have, and can’t have/couldn’t have.
Compare:
She must be really exhausted from working all day. (present deduction)
She must have been very exhaused when she had to work all day. (past deduction)
must have
Must have + past participle is used to express certainty about something.
She couldn’t find her purse. She must have left it at the restaurant.
Allan must have had the flu. He didn’t show up at any of his appointments today.
We’ve got enough money. Michelle must have saved on her meal earlier.
might have / may have
Might have or may have + past participle is used to express the possibility that something happened.
I might have bought the wrong brand. Can you please ask Mom to check?
Joe thinks his wife may have left with her lover.
Note: Might have is the less formal form compared to may have. Could have is another possible form to be used in such context but it is not that very common.
can’t have / couldn’t have
Can’t have and couldn’t have + past participle are used to express that it is not possible for something to happen.
They can’t have left the party yet because their car is still outside.
Andrea thought she saw her old university professor this morning but it couldn’t have been her – she moved abroad years ago.