A preposition is a conditional term used in a sentence, creating syntax and context. The use is further qualified by tense and content, and may contain additional qualifiers, expressing further conditions in context as added prepositions in the form of conjunctions. (This is if/then logical structuring in sentences.) Use of prepositions is often based on existing subject matter, so usage is also associative within the broad context of the topic.
Examples of Preposition:http://owl.english.purdue.edu/owl/resource/594/01/