The document discusses perfect modal verbs, which combine a modal verb like "should", "could", etc. with the auxiliary "have" and a past participle verb. It explains that the meaning depends on the modal verb used - "should" expresses regret about past actions, while "would", "could", "may", "might" speculate about the past. Examples are provided like "I should have studied more" expressing regret and "I would have been happy there" speculating about a past possibility.