How do you tell if a word is a preposition or an adverb?

Answer 1

Look for an object: a noun or pronoun or the equivalent which normally follows the preposition. If there is none, the word in that usage is an adverb.

Example:

We walked on. (adverb)

We walked on the beach. (preposition)

Sometimes the object is separated from its proposition.

- Who (whom) did she see her boyfriend with?

The object whom is the object of the preposition with.

- Which war did he serve during?

The object is which war: he served during which war?

Answer 2

Prepositions typically show a relationship between a noun/pronoun and another element in a sentence, while adverbs modify verbs, adjectives, or other adverbs. To determine if a word is a preposition or an adverb, check if it is followed by a noun/pronoun (preposition) or if it modifies a verb/adjective/adverb (adverb).