What is an infinitive in grammar?

An infinitive in grammar is defined as the combination of the word "to" followed by the base form of a verb. This construction is commonly used in English to express action in a non-finite form, meaning that it does not convey the tense on its own and can function in various capacities within a sentence. For example, in the phrase "to run," "to" is the infinitive marker, and "run" is the base verb. Infinitives can serve as nouns, adjectives, or adverbs, which showcases their versatility in sentence structure.

The other options present different grammatical concepts. One refers to verb forms that indicate past tense, which specifically involves conjugation rather than the use of "to." Another option describes a noun that represents a subject, identifying a distinct grammatical category, while the last option refers to a descriptive term for an action but does not capture the specific function of infinitives. Thus, the correct identification of an infinitive is centered on its characteristic structure, linking "to" with the base verb.