Noun

• To play is fun. [Noun; subject of the verb is. To prove that it's a noun, we can change the infinitive to a gerund: Playing is fun.]

• I love to eat. [Noun; direct object of the verb love. To prove that it's a noun, change the infinitive to a gerund: I love eating.]

✔ TIP: when a main verb (the verb that comes right after the subject) has an object that is a noun or a pronoun, it is usually followed by an infinitive and not a gerund.

Adjective

• I had a car to offer. [Adjective modifying car. The fact that it's modifying a noun makes it an adjective. To prove this, turn it into an adjective clause: I had a car that I offered.]

✔ TIP: infinitives are usually used after an adjective. A gerund is sometimes possible, but usually an infinitive is better.

Adverb

• Dave walked to meet her. [Adverb modifying the verb meet. To prove it this time, turn it into an adverbial clause: Dave walked so he could meet her.]

✔ TIP: unfortunately, there are times when either a gerund or an infinitive can follow the main verb in a sentence. There are no rules in this case and second language learners must memorize the most common verbs that take an infinitive.