adverb
An infinitive almost always begins with “to” and is followed by the basic form of the verb. It is usually in this form:
To + Verb = Infinitive
An infinitive is NOT a verb, so s, es, ed, or ing should not be added to the end. Infinitives can be used as nouns, adjectives or adverbs. According to the pattern of what an infinitive should be, “to cook” is the infinitive of this sentence and it functions as a noun.