I don't accept the offer but receive an email about my acceptance?

[alexxu]

Hi! I am a student who receive the admission and TA letter from George Mason University. The TA offer letter is

" If you have any questions, please contact the Director of the PhD program, Prof. Sanjeev Setia at csphd@gmu.edu.
 Please indicate your acceptance of this appointment by sending an email message to csphd@gmu.edu as soon as possible. "

So I sent an email to csphd@gmu.edu to ask about the deadline of making my decision. 

But today I receive an letter which wrote

"on your acceptance to the PhD program of the George Mason University Computer Science department "

But I don't accept the offer. So what should I Do?

Edited February 10, 2017 by alexxu

[bhr]

1 hour ago, alexxu said:

Hi! I am a student who receive the admission and TA letter from George Mason University. The TA offer letter is

" If you have any questions, please contact the Director of the PhD program, Prof. Sanjeev Setia at csphd@gmu.edu.
 Please indicate your acceptance of this appointment by sending an email message to csphd@gmu.edu as soon as possible. "

So I sent an email to csphd@gmu.edu to ask about the deadline of making my decision. 

But today I receive an letter which wrote

"on your acceptance to the PhD program of the George Mason University Computer Science department "

But I don't accept the offer. So what should I Do?

You are misreading them. Acceptance seems like it means their acceptance of your application, though I would need to see the rest of the context to be clear. Have you actually declined their offer yet?