An infinitive is the base form of a verb, often preceded by the word "to." For example, in the phrase "to run," "run" is the infinitive. Infinitives can function as nouns, adjectives, or adverbs in a sentence, making them versatile in language. Infinitives can also appear in different forms, such as the "bare infinitive," which omits "to," as in "I can swim." Additionally, infinitives can be used in phrases, like "to eat healthy," where they express purpose or intention. Understanding infinitives helps in constructing clear and effective sentences.