These terms can indicate the direction, time, location or introduction of an object. For example, in the sentence, "The neighbor is at the door," at is the preposition indicating location.

But it deals with time the same way, so it’s an adverb. “For” is a preposition. Prepositions take objects, which are always either nouns, pronouns or whole phrases or clauses working as nouns.

These terms can indicate the direction, time, location or introduction of an object. For example, in the sentence, "The neighbor is at the door," at is the preposition indicating location.