ome words can be used as either prepositions or adverbs. If the word has an object, it is acting as a preposition. If it has no object, it is acting as an adverb.

Adverb: My school bus just went past.
Preposition: My brother just drove past us.

Hint:Ask what after the word to see if it has an object.

Adverb: My school bus just went past. (Past what? Nothing = Adverb)
Preposition: My brother just drove past us. (Past what? Us = Preposition)