Hi src_saurav,
Since you're asking about idioms, it sounds like you're interested in improving your SC skills. While there are 100s of idiomatic phrases (as well as 'usage' and 'style' phrases) in the English language, the GMAT tends to use certain phrases more often than others.
Now, before you go about trying to memorize lots of different idioms, you have to make sure that your overall grammar knowledge and pattern-matching skills are solid. Most SCs are based on 2-4 grammar rules, and the good news is that you don't normally need to know all of the rules involved to get the correct answer.
How are you currently scoring on your practice CATs?
How are you performing on SCs overall?
GMAT assassins aren't born, they're made,
Rich
