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. They are versatile and can express actions, intentions, or purposes. 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 be or not to be," which is a famous line from William Shakespeare's play Hamlet.