There's a huge difference between your and you're. If you're going to your friend's house, your not going to you're friend's house. Get it?
And it's must have, not must of. As in I must of left it somewhere. Wrong! You must have left it somewhere.
It's easy to see how must have becomes must've which sounds like must of. But a growing, developing language is a whole different thing to laziness.
And don't even get me started on texting! It will undermine the language in ways we don't even understand yet. I do it too and every time I use a stupid shortcut I hate myself for doing it.
Okay, pet peeve session over.