Hi apolo,
You'll likely get more of a response if you post this in the SC Forum:
gmat-sentence-correction-sc-138/As you continue to study, you're going to find that the GMAT focuses on a specific sub-set of grammar rules, so you don't need to have every little grammar rule memorized. However, you DO need to know the major grammar rules and idiom/style rules. As you work through your GMAT practice materials, you'll likely find it beneficial to read the explanations to EVERY SC that you do (even the ones that you get correct). As you come across minor/rarer rules (or rules that you did not know), you should write them down and keep a master-list of the little rules that you come across. In that way, you can revisit that list periodically to make sure that you still remember the rules that the GMAT might test you on.
GMAT assassins aren't born, they're made,
Rich