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 usage. 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," which express intentions or purposes. Understanding infinitives helps improve sentence structure and clarity in writing.