what is a preposition

A preposition is a word or group of words used before a noun, pronoun, or noun phrase to show direction, time, place, location, spatial relationships, or to introduce an object. Prepositions and postpositions, together called adpositions, are a class of words used to express spatial or temporal relations or mark various semantic roles. A preposition typically combines with a noun phrase, which is called its complement or object. A preposition comes before its complement, while a postposition comes after its complement. Prepositions are highly idiomatic in English, and although there are some rules for usage, much preposition usage is dictated by fixed expressions. Some examples of prepositions are words like "in," "at," "on," "of," and "to".