Prepositions
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. Some examples of prepositions are words like "in," "at," "on," "of," and "to."
Prepositions in English are highly idiomatic. Although there are some rules for usage, much preposition usage is dictated by fixed expressions. In these cases, it is best to memorize the phrase instead of the individual preposition.