Thanks! That is actually a clear way to distinguish between the two. I was also sent an email regarding that, I apologize, for the link was not given to me... Here is an excerpt from my source:
Here is the difference:
Many of the words in the preposition list can also be used as adverbs. The problem for you is to figure out when a word from the list is being used as an adverb or as a preposition. When in doubt, ask the questions whom or what after the word. If there is a noun or a pronoun to answer the question, then the word is a preposition with an object--a prepositional phrase. If there is not a noun or pronoun to answer whom or what, then the word you are worried about is an adverb.
Example: The girl looked over and then ran down the street.
Both the words "over" and "down" are on the preposition list. Say the word "over" and add "over what?" There is no what or whom word after "over." Now say "down what?" -- "down the street." There is a noun "street" which tells what after the word "down." Therefore, "down" must be a preposition with the prepositional phrase being "down the street."
Just my 2 cents worth. =)
In additiion, the phrase down the street is headed by the preposition down, true, but the entire phrase functions as an adverbial as it answers the question where:
Q: Where did she run?
A: Down the street. [adverbial phrase]