An infinitive is formed with the word "to" plus the base form of a verb. It can function as a noun, subject, direct object, adjective, or adverb. Common examples are "to sing", "to dance", and "to go". An infinitive phrase includes an infinitive plus any other words needed to complete its meaning, such as "to fly a plane". The document provides examples of infinitives and infinitive phrases, and instructs students to identify examples from a reading assignment.