Infinitive phrase

An infinitive phrase is the plain form of a verb introduced by 'to', plus any objects or modifiers hanging off it — 'to reach the summit before dark' — used as a noun, as an adjective, or most often as a statement of purpose.

He took the earlier train to reach the summit before dark. The phrase runs from to through dark and answers why, which is the job infinitive phrases do best. The alternative is a whole subordinate clause — so that he could reach the summit before dark — and the infinitive gets there in four fewer words.

It shifts role depending on where it sits.

• Subject: To be doubted was new to her.
• Object: She wanted to be doubted.
• Adjective: A reason to stay.
• Purpose: She stayed to be doubted a little longer.

Two working notes. First, splitting the infinitive — putting a word between to and the verb, as in to quietly leave — has never actually been an error in English, and rearranging a sentence to avoid it usually makes the sentence worse. Second, a run of infinitives gets tangled fast: She agreed to try to get him to promise to call. Each one is fine; four is a knot.

Like the gerund phrase, an infinitive phrase is a verb doing a noun's work, which is why it can carry an object of its own and still slot into a sentence as a single piece.