As defined in the dictionary on my Mac (circle R), a preposition is a “word governing, and usually preceding, a noun or pronoun and expressing a relation to another word or element in the clause…” snore.
A preposition is the word that places the subject. Think of an apple on a table. The preposition is going to tell you wherever that apple is in relation to the table: under, on, beneath, beside, etc. Prepositions are like the Facebook status updates for the subject of a sentence. Apple is so over that table.
I bring this up because I want to share with you the preposition song that I still hum to myself to determine whether or not something is a preposition. It goes to the tune of Yankee Doodle Dandy:
With, on, for, after, at, by, in, against, instead of, near, between, through, over, up, according to, around, about, beyond, into. Until, within, without, upon, from, above, across, along, toward, before, behind, below, beneath, beside, during, under.
That was much less satisfying to type out than to sing. In any case, enjoy!