In the sentence "I want to sleep when I'm tired," the words "to sleep" are a/an

A. predicate adjective.

B. indirect object.

C. prepositional phrase.

D. infinitive phrase.

In this sentence, the words "to sleep" are an D. infinitive phrase. This is because the words start with an infinitive to sleep, which takes the form of to + verb. It isn't an adjective, but a verb, so it cannot be a predicate adjective. It is a direct object, not an indirect one. It isn't a prepositional phrase, because it doesn't start with a preposition.