Grammar

In my opinion, I have pretty decent grammar. I think it is just ridiculous to brag about being a grammar nazi because not everyone can remember all of the rules all of the time. Then, the first time you forget, someone else slams you and you are thusly ousted from your grammar thrown. I almost always remember to say "it's" when I mean "it is" and "its" when I mean possessive it. I fully comprehend all of the differences between your/you're. I cannot for the life of me figure out/remember the who/whom jig. What is the deal?