You can't have an affirmative contraction before a gap, although you can have a negative one.
You are so beautiful tonight.
How beautiful you are __ _______ tonight!
*How beautiful you're tonight!
__________
You are smart and I am smart, too.
You are smart and I am ____, too.
*You're smart and I'm, too.
You're smart, but I'm not.
You're smart, but he's not.
You're smart, but he isn't.
_________
He is taller than you are (tall).
He is taller than you are ____.
*He's taller than you're ____.
________
I'll go to the concert, but you won't __ __ ___ ______.
*I won't go to the concert, but you'll __ __ ___ _______.
I won't go to the concert, but you will __ __ ___ _______.
(Recall that * indicates an ungrammatical sentence.)
CJ