Yes yes, I know this is a pretty obvious choice for G, but you know what they say, it's a classic for a reason.
There are always some of the lucky few that have a natural feel for grammar and the other rules. Others don't. Luckily, grammar is an easy (if repetitive and tedious) fix once you have an idea of the rule.
However, because there are so many rules, I'm not going to go into them today. Rather, I will refer you to this lovely lady.
Lucy is also doing the A to Z Challenge and her theme this year is Writing Conventions. Each letter deals with a specific grammar or writing rule.
I also suggest you go check out Grammargirl. She probably has every single thing to know about grammar on her site.
But before you turn yourself into a grammar nazi, remember that the prerogative is still yours. If you want to break a grammar rule for a reason, do it. As long as it works stylistically (ask crits and betas) and the reason isn't because you're too lazy to fix the mistakes.
So what about you? Grammar Nazi, or Grammar Hippie? Or somewhere in between? Have you ever experimented with breaking grammar rules? How did it work out?