Hi vishal1981,

In SCs, there are 5 major grammar rules (parallelism, verbs, pronouns, modification, comparisons) and a lump category called usage/style/idioms. Most SCs are based on 2-4 grammar rules, and you usually only need to know 2 of the rules to answer the question correctly. Unfortunately, a number of SCs will come down to usage, style and idiom rules. Certain rules are more frequently tested than others, but you won't know exactly which ones are on your GMAT until you see them. Knowledge is a powerful thing - knowing your idioms can help you to pick you up a bunch of points on Test Day, so there's an argument for learning those rules. Think of all of the math rules, formulas, content, number properties and tactics that you know already (you memorized those rules), so it shouldn't be that much trouble to learn your idioms. My advice is to learn idioms in small chunks and build up your knowledge over time.

GMAT assassins aren't born, they're made,
Rich