The document discusses modal verbs and their main features. Modal verbs are followed by the infinitive form of another verb without "to", don't take verb endings, and are never used with other auxiliary verbs. They can express ability, obligation, prohibition, certainty, possibility, and advice. The modal perfect tense uses modal verbs with "have" and a past participle to talk about past ability, certainty, possibility, impossibility, and criticism of past actions.