This document discusses the use of modal verbs (must, may, might, can't, could, should, shouldn't) followed by the verb "have" and a past participle to talk about certainty, possibility, impossibility, alternatives, and criticism regarding past events. Specifically, it explains that: 1) "Must have" expresses certainty about something that happened in the past based on logic or deduction. 2) "May/might/could have" suggests possibility but uncertainty about something that happened in the past. 3) "Can't have" indicates something is 100% impossible or untrue about the past. 4) "Could have" refers to alternatives in the past that