Hi kdn508,

Getting good at SCs requires a mix of content knowledge (knowing grammar rules and idioms) and tactics (using the answer choices to your advantage, spotting the common errors that the GMAT "builds" its SCs around, etc.).

As with any content area, it's important to start with the basics/essentials, then make note of the rarer rules as you come across them. Keeping an ongoing list of those rarer rules can help you as you're reviewing past material. You might also find it useful to read the explanation for EVERY SC that you do, taking note of minor rules that you didn't realize were in the prompt.

Most SCs are based on 2-4 grammar rules each; the good news is that you usually DON'T need to know all of the rules to find the correct answer. You DO need to be tactical though. Picking an answer because it "sounds right" is the LAST tactic you should use (in some situations, it's sometimes the worst way to go about answering an SC). It's what you should do when you're out of options.

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