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 and memorizing those. Non-native speakers unfortunately will have to memorize the most common idioms.
The second type of idiom is the connection of a verb to a preposition to create the proper meaning, for instance you DISTINGUISH one thing FROM another, you are PROHIBITED FROM doing something but I FORBID you TO do something. These also need to be memorized. The difference between these types and the type above is that these type focus on the verb, while the above type does not.
Idioms are among the most difficult items on Sentence correction, especially for non-native speakers. Generally, I advise my students to look for other errors in the sentence and deal with any idiom problems when you have as few answer choices as possible left. Then I suggest the student identify the word attached to the "little word" (which is often a preposition) and simply say the combination to themselves instead of repeating the answer choices - often the answer choices simply confuse the issue further.
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