The prepositional phrase includes the preposition and the object of the preposition as well as any modifiers related to either. The flying saucer appeared above the lake before it disappeared into space. Crystal could hear her sister snoring across the room .
Objects usually answer the question what.
Christine discovered a pile of books hidden under the staircase . You should consider reading the notes before class. You should consider reading the notes before you come to class. Alix walk down the ramp to the beach. DOWN is not an adverb because it alone does not answer where about the verb.