Modal Perfects

They express possibility or probability. When used with the infinitive form of the perfect ("have" + past participle), modal verbs indicate speculation about things in the past.

"Must have" is used to express a logical conclusion about something that happened in the past We are not sure exactly what happened, but based on the evidence, we can come to some conclusion, unlike "might have" or "may" have ", as we will see later," must have "expresses more certainty.

"May have" and "might have" are used to express possibility in the past. The use of these two manners also expresses uncertainty.

"Can not have" is used in a similar way to "must have", but in the negative way. We can use "can not have" when we are fairly certain that something did not happen or that it was not true in the past.

"Should have" and "ought to have" is used when something did not happen, but it would have been better if it had happened. In the negative, these referents indicate that something happened, but it would have been better if it had not been like that.