The document discusses complementary infinitives in English and Latin. In English, an infinitive is "to" plus the verb and some verbs require an infinitive to complete their meaning, like "want to play". In Latin, the infinitive is the second principal part of the verb from the dictionary and all regular infinitives end in "-re". Like in English, some Latin verbs need a complementary infinitive, such as "possum" which is often followed by an infinitive to fully express an idea, like "I am able to climb a tree" translates to "Audīre non poteramus".