An infinitive consists of the preposition to and a verb. They serve as nouns, adjectives or adverbs.
To forgive is admirable. (To forgive is an infinitive acting like a noun which is the subject of the sentence.)
She wants to study. (To study is an infinitive acting like a noun which is the direct object).
I don't have time to waste. (To waste is an infinitive acting like an adjective which modifies the noun time.)
He was invited to speak. (To speak is an infinitive acting like an adverb which modifies the adjective invited.)
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