If I drop out of a PhD program, do I have to pay back the university? I recently accepted a PhD program offer of roughly $21,000 for a year (2 semesters) of TA work.  I will be attending a university in the USA, and I am studying computer science, if that affects the answer.  If I drop out before going through the whole program, do I have to pay anything back?  My girlfriend was concerned about this.

I already looked for a duplicate answer on Academia Stack Exchange but didn't find any.
It would be almost unheard of in the US for you to be asked to pay back money in such an arrangement. But note that you are probably paid monthly, and if you quit in the middle of a year, the monthly payments will stop then, and you won't see the entire amount. 

You aren't promised the total, but only an amount that will come to that total if you see it through.