Nǐ yǐwéi nǐ shì shéi?
你以为你是谁?
Who do you think you're?
Notes:
你(nǐ): you
以为(yǐwéi): to think
是(shì ): to be(am, is, are)
谁(shéi): who
Example:
A:你以为你是谁!
Nǐ yǐwéi nǐ shì shéi?
Who do you think you're?
B:我是的老婆!
Wǒ shì nǐ lǎopo!
I'm your wife.