Hi ankban,
In general, each SC that you'll face on the GMAT will be based on 2-4 grammar rules. Thankfully, there aren't that many grammar rules that you need to know for the GMAT (although there are lots of idioms that could potentially appear on your Exam). The issue that you're facing is that you're not actively looking for which grammar rules are there (or NOT there), so you're relying on how the answer choices "sound" (and that's not a particularly great way to work through SCs - it's what you do when you have no other option). Thankfully, SCs are a remarkably consistent aspect of the GMAT, so you can train to spot the common patterns (re: how the GMAT writers specifically test certain grammar rules) and pick up a lot of points in the Verbal section.
1) How long have you studied so far? How many hours do you typically study each week?
2) What study materials have you used so far?
3) On what dates did you take EACH of your CATs/mocks and how did you score on EACH (including the Quant and Verbal Scaled Scores for EACH)?
4) What is your overall goal score?
GMAT assassins aren't born, they're made,
Rich