Form
S + modal + have + V3
S + modal + not + have + V3
I should have studied harder.
He might have been asleep.
You shouldn’t have eaten so much.
Meanings
Past Possibility & Speculation
We use might have, may have, and could have to speculate or make a guess about a past event. We are not sure what happened.
might have / may have: A past possibility. (e.g. 30-50% certain)
“Why didn’t he call?” “He might have forgotten.”
could have: Suggests one of several possibilities. Also used for a past opportunity that was not taken.
(Speculation): The police are not sure how the fire started. It could have been an accident.
(Lost opportunity): You could have gone to university, but you decided to travel instead.
couldn’t have: Certainty that something was impossible in the past.
She couldn’t have stolen the money; she was with me all day!