If I had the chance to change one thing about English grammar it would be that wretched rule about "it's" and "its"! I know that the apostrophe indicates a contraction of "it is" but I often find that I'm thinking of the possessive sense of the word as I write (as in "belonging to it") and applying an invalid rule. Then I lapse into using "it's" when I should have used "its". On more than one occasion I've reread some email I've already sent out to half the company, spotted this mistake too late and felt cross with myself for letting it out of the door. I know no one ever said that English grammar was logical but there is something about this particular rule that really annoys me!