Prepositions govern nouns or pronouns. That is, prepositions are followed by nouns or pronouns. Prepositions go together with nouns or pronouns. Prepositions form a unit of meaning with the nouns or pronouns that follow them.
Some words which can be used as a preposition can also be used as an adverb. You will know whether the word is a preposition or an adverb by what follows. If