An infinitive is a phrase, consisting of the word to and the basic form of a verb, that functions as a noun, an adjective, or an adverb. Here’s a discussion of the five types of infinitives.
An infinitive can constitute the subject of a sentence. For example, in “To go, even after all that trouble, didn’t seem worthwhile anymore,” “to go” is the action that drives the sentence.
2. Direct Object
In the sentence “We all want to see,” “to see” is the direct object, the noun (or noun substitute) that receives the action of the verb. “To see” refers to a thing being done — or, in this case, desired to be done: the act of seeing.
3. Subject Complement
In “My goal is to write,” “to write” is the subject complement. A subject complement looks just like a direct object, but the difference is in the type of verb preceding it. The verb in the previous example, want, is a transitive verb. (Transitive verbs have two defining characteristics: They precede a direct object, and they express an action.)
In “My goal is to write,” the verb is a copular, or linking, verb — one that links a subject to a word or phrase that complements it. (In this sentence, “to write” is the goal, so it’s the complement of goal. Note that in the previous example, “to see” is what those referred to as we want, but it’s not the complement of we.)
In “She didn’t have permission to go,” “to go” modifies permission — it describes what type of permission is being discussed — so the phrase serves as an adjective.
In “He took the psychology class to try to understand human behavior,” “to understand (human behavior)” explains why the taking of the class occurred, so it’s an adverb modifying the verb took.