Idiomatic expressions are phrases that are common in standard written English. There is no logic behind their usage. You either know them or not. You will have to use your ear most of the time. But its highly advised that you learn the list of common idioms on GMAT.
I am sorry to tell you this, but memorizing them is the only way you can do this. Start doing this early in your preparation. Initially it’s okay to look up the idiom list while answering the questions. Watch out for the changing prepositions while doing a vertical scan of the answer choices, this would give you a hint that may be this is an Idiom error.
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