|The Structure of English Language Prepositions|
Prepositions are words or groups of words, that introduce phrases; and these phrases modify some element in a sentence. What follows a preposition is normally a noun, pronoun, or noun clause. A word that follows a preposition is its object, and, in the case of pronouns especially, this affects the form of the word.
One of the problems in spotting prepositions in a sentence is that many of the words that are usually prepositions can also be used as adverbs.