This document defines and explains the use of the to infinitive in English grammar. It begins by defining the to infinitive as a verb form consisting of "to" followed by the base form of the verb. It then outlines several common patterns involving the to infinitive, such as subject + verb + to infinitive. The document explains that the to infinitive can serve different grammatical functions, including as the object, subject, subject complement, and complement of nouns and adjectives. It provides examples to illustrate each of the uses and functions of the to infinitive in English.