I ate a piece of your cake.
✗ I ate a piece of you're cake.
you're going to buy a cake, right?
✗ your going to buy a cake, right?
"Your" is used to refer to a possession of the person you are talking to.
Please take your bag with you.
The bag belongs to you.
"You're" is a shorter form of "You are".
You're too tall to sit comfortably here.
Equivalent to: You are too tall to sit comfortably here.
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