An idiom is a combination of words that seems perfectly natural to the native speaker of a language but seems odd or peculiar to other people (usually because it has a meaning different from the literal meaning of the words).
Idiomatic expressions generally consist of nouns, verbs, adjectives, and phrases that are associated with specific prepositions. If any other preposition is used, then the expression is unidiomatic and, therefore, incorrect. Below is a list of the “Idiomatic Use of Prepositions.” It is impossible to list all of the prepositional combinations, but several of the most troublesome are given below. When in doubt about the appropriate preposition for a particular word or expression, consult a dictionary.
You live in America but on the land.
You live on the farm but in the city.
You live on earth but in the world.
You may be secretary to the Dean or of the Dean.
You may go uptown or downtown.
You may go out on the town but never out on the city.
You may be in bed or on the couch but seldom in the bed and never in the couch.
You may be interested in music without an aptitude for music.