The third verbal is the infinitive: to plus the base form of a verb (to win, to read, to help). After participles (adjectives) and gerunds (nouns), the infinitive is the busiest: it can do three jobs.
| Job | Example | Test |
|---|---|---|
| noun | To win was her goal. Leo wants to learn the guitar. | Replace it with something. |
| adjective | We need a book to read. | It tells which or what kind of noun. |
| adverb | She came to help. The puppy was eager to play. | It tells why, or modifies an adjective. |
Key idea
An infinitive is to + a verb. To + a noun is a prepositional phrase, not an infinitive.
Why it works
Infinitives let you name goals and purposes: what someone wants to do, why someone came, what something is for. The to + verb pattern is also why some readers object to a “split infinitive” (to boldly go). Most usage guides today accept splitting when it reads better; that debate comes up in Usage and Mechanics.
Worked example: Infinitive or prepositional phrase?
“We walked to the park to play soccer.”
- to the park: to + a noun (park). A prepositional phrase.
- to play soccer: to + the verb play. An infinitive phrase, telling why we walked: used as an adverb.
Answer: infinitive phrase: to play soccer
Worked example: Name the job
“My plan is to finish early.”
- Replace it: “My plan is something.” That works.
- So to finish early is used as a noun (a predicate noun after is).
Answer: noun
Common mistake
Mixing up infinitives and prepositional phrases because both start with to. Look at the next word: a verb (to run) makes an infinitive; a noun or pronoun (to the store, to her) makes a prepositional phrase.