An Idiom is a phrase or a way of saying things that is accepted useage but may not be understandable from deciphering the individual words. On the GMAT this boils down to two types of idioms. There are phrases that require certain words for instance: "Such as", "more... than", "so... that" These type of idioms need to be memorized. Native english speakers probably know about 90% of the idioms and need to focus on discovering which idioms they have used incorrectly
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