What must an infinitive phrase contain?

An infinitive phrase is defined by the presence of the word "to" followed by a base form of the verb, which is also referred to as an uninflected verb. This structure is essential as it signifies the phrase's function as a noun, adjective, or adverb in a sentence. The infinitive phrase may include additional elements such as objects or modifiers that provide more context, but the core requirement is the combination of "to" plus the simple form of the verb.

For example, in the phrase "to run quickly," "to" is paired with the verb "run," creating an infinitive phrase that can perform various functions within a sentence. This fundamental characteristic distinguishes infinitive phrases from other types of phrases that may require more complex elements, such as subjects or auxiliary verbs.