It's / Its

Sometimes I feel like it was a complete waste of time bothering to learn any grammar at school, I can't help but feel that it would be nicer to simply not know the difference between it's and its so that it didn't get on my sodding nerves so much when I read web pages which are strewn with examples of incorrect usage of the two. The amazing thing is that some people consistently get it wrong by using "it's" when they mean "its" and then, later on the same page, using "its" when they mean "it's". Here's the rule (for any of you who don't know): "It's" is a contraction of "it is" or "it has". "Its" is a possessive pronoun, like his. You wouldn't write "hi's", would you?

It's not even as if I learnt a great deal of grammar at school, the place was terrible when it came to teaching English, but even with that handicap I can still see the difference between the two. It's bloody obvious.

And it's not "could of" either, that's another bloody annoying one. It's "could have". "Could of" presumably started popping up because people hear "could've" and mistakenly think they're hearing "could of", when it is in fact a contraction of "could have". The same goes for "would of", it's "would have".

Last but not least, of course, is "there", "their" and "they're":

I could go on but why bother when I can just refer the world to this article on WikiPedia? I do however know that I am preaching to the converted... It just gets on my tits, that's all.