An infinitive is a specific type of verbal phrase that stands out in a sentence because of its predictable construction. It contains a verb in its simplest, present tense form, and is usually preceded by the preposition "to." Though it is easy to spot the infinitive, it is harder to know which part of speech it is functioning as in the sentence since it can act as a noun, adjective or adverb, according to Purdue.edu.
Since an infinitive is a type of verbal, like a gerund or a participle, it means the phrase contains a verb, and it is emphasizing some particular state of being. Yet, an infinitive does not need to function as the main verb, or action, in a sentence. It can function in many positions within a sentence including a subject, adjective, adverb, direct object and subject complement.
Sometimes, when the infinitive acts as a direct object, the "to" gets dropped from the phrase. "The assistant helped teach the class grammar" is an example of an infinitive in the direct object slot.
Other examples include the more standard version of the infinitive which contain the word "to" before the simple, present tense form of the verb. "To wait seemed irresponsible with the report due at the end of the day." This previous sentence highlights an example of the infinitive acting in the subject position.
As an adverb, the infinitive looks like the sentence, "the baby cried to get our attention."
These are just a few examples of the ways the infinitive functions in a sentence, but the construction, definition, and various examples of the phrase are good things to remember in order to learn ways of identifying this verbal.