The document discusses perfect modal verbs, which are formed using a modal verb like "should", "could", "may", etc. followed by the auxiliary "have" and a past participle verb. It provides examples of forms like "should have studied" and explains that the meaning depends on the modal verb used. Specifically, "should have" expresses regret about past actions not happening, like failing a test one should have studied more for. The document outlines the grammatical structure and meaning of different perfect modal constructions.