An infinitive phrase is a group of words that begins with an infinitive, which is the base form of a verb preceded by the word "to." For example, in the phrase to run a marathon, "to run" is the infinitive, and "a marathon" is the object of the verb. Infinitive phrases can function as nouns, adjectives, or adverbs in a sentence. These phrases can add detail and clarity to writing. For instance, in the sentence "She wants to learn Spanish," the infinitive phrase "to learn Spanish" acts as the object of the verb "wants." Infinitive phrases can also express purpose, as in "He studies hard to pass the exam."