How can you tell if a word is a preposition?

Answer 1

A preposition is followed by a noun (or pronoun, or gerund, or noun phrase) and does not introduce a complete thought, i.e. there is no conjugated verb in a prepositional phrase. The noun is called the object of the preposition, and helps to define the noun, verb, adjective, or adverb that the phrase modifies.

Preposition:

He left before the dance

Conjunction:

He left before the dance was over.

Answer 2

A word is likely a preposition if it is followed by a noun or pronoun and indicates the relationship between that noun or pronoun and another element in the sentence. Common prepositions include words like "on," "in," "under," and "with."