[In the following examples, the preposition is bold and the prepositional phrase is underlined.]

• The flying saucer appeared above the lake before it disappeared into space.
  • ABOVE is not an adverb because it has an object to complete its meaning; therefore, ABOVE is a preposition and the entire phrase is an adverb phrase.
    
    
• Crystal could hear her sister snoring across the room.
  • Objects usually answer the question what. Therefore, we can ask across what? to determine the object of the preposition.
    
    
• Christine discovered a pile of books hidden under the staircase.
  • UNDER is not an adverb because it alone does not answer where about the verb.
    
    
• You should consider reading the notes before class.
  • BEFORE is not an adverb because it alone does not answer where about the verb.
    
    
• You should consider reading the notes before you come to class.
  
  • BEFORE is not a preposition because is not followed by an object that it links to the clause. It is followed by another clause that is subordinate in meaning to the independent clause; therefore, it is a subordinate conjunction.
    
    
• Alix walk down the ramp to the beach.
  
  • DOWN is not an adverb because it alone does not answer where about the verb.
    
    
• Alix fell down.
  
  • DOWN is an adverb answering where about the verb. There is no object, so it cannot be a preposition.