Question

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

Answer 1

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.