Your | You're
This is one of the classic homonym traps, and every grammarian and proofreader will have stories (of horror or amusement!) of the clangers that they have had to endure from more careless writers.
- Your is the possessive form of you.
- You're: is a contraction of 'you' and 'are'.
As you can see, the contraction is causing problems once more. As with it's/its and who's/whose, the solution is extremely simple. Put the 'are' back into the sentence in full, and it's immediately obvious which form to use:
- You are always welcome here.
- CORRECT: You're always welcome here.
- INCORRECT: Your always welcome here.
- You are not spelling your words correctly
- CORRECT: You're not spelling your words correctly.
-
INCORRECT: Your not spelling you're words correctly.
Your brain is the most powerful tool you have when you write. Following these simple rules, you are showing your audience that you're a careful writer who doesn't just throw down the first spelling that comes to mind when writing. That's good for you - a frustrated, appalled or even just amused reader is no good to you, but a rapt one?