Parts of speech are also used as or made into adverbs.

(a) Nouns - Inside, outside, home-made, skin-deep, sea-borne.

(b) Adjectives - Pretty good. He works hard.

(c)  Pronouns - None the worse. All the better.

(d) Prepositions - You go before, I will come after. I think he will pull through.

Note: To tell when a preposition is used as an adverb see whether it is followed by a noun. If not, it must be parsed as an adverb.

A cloth must be put underneath (adverb).

A cloth must be put underneath the books (preposition).