The document discusses the perfect infinitive in English and Spanish. In English, it is formed with "to have" and the past participle, expressing that the action occurred in the past. Similarly, in Spanish it uses "haber" and the past participle. It can follow certain verbs in past or present tense. The document also discusses nuances between using "ser" and "estar" in Spanish, particularly with the verb "morir", where "estar" expresses being dead and "ser" expresses having been killed.