Definition
A preposition is a word or set of words that indicates location (in, near, beside, on top of) or some other relationship between a noun or pronoun and other parts of the sentence (about, after, besides, instead of, in accordance with). A preposition isn't a preposition unless it goes with a related noun or pronoun, called the object of the preposition.
Examples:
Let's meet before noon.
Before is a preposition; noon is its object.
We've never met before.
There is no object; before is an adverb modifying met.
Rule 1. A preposition generally, but not always, goes before its noun or pronoun. One of the undying myths of English grammar is that you may not end a sentence with a preposition. But look at the first example that follows. No one should feel compelled to say, or even write, That is something with which I cannot agree. Just do not use extra prepositions when the meaning is clear without them.