Hey BlitzHN,
I'm always a little nervous when I see a post title like yours! I've long believed (and had my theories confirmed time and time again) that while you
could know those hundreds of idioms, you don't have to. Much like you
could solve a quant problem by multiplying together 3 or more 3-digit numbers, but the GMAT is well-written enough that it will reward you for using number properties or divisibility rules to solve it without the extra work.
Some of my favorite GMAT problems are those that trap people into trying to use an idiom, but on which the correct idiom is one that people don't recognize as correct. On these problems, the idiom folks tend to miss a much bigger-picture error. Consider this one:
Immanuel Kant's writings, while praised by many philosophers for their brilliance and consistency, are characterized by sentences
so dense and convoluted
as to pose a significant hurdle for many readers interested in his works.
A. so dense and convoluted as to pose
B. so dense and convoluted they posed
C. so dense and convoluted that they posed
D. dense and convoluted enough that they posed
E. dense and convoluted enough as they pose
Hint: You won't like the correct idiom, and you'll probably eliminate the wrong answer without ever considering the verb tense situation at the ends of the answer choices (pose vs. posed).
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My recommendation - as you practice Sentence Correction, try to find the major errors first, and save idioms for the end. We're simply not that good at knowing/memorizing all of those idioms, and by definition the lists that any company or website has aren't completely comprehensive...there are thousands of them out there. The GMAT - man, I'd love to say always but there may be a few isolated exceptions - almost always gives you the opportunity to make a decision on something not-idiomatic that's usually an easier way to solve the problem anyway.
Try this - do a set of 20 problems from the
OG or whatever book you have and write down your reasons for eliminating answer choices. If you say "Idiom" more than 10 times, go back to those and force yourself to find a more systematic reason. Train yourself to think systematically and not idiomatically, and the test becomes much, much more manageable.