I enterd the United States in 2006 and I did my Masters but also taught for 2 years as a teaching assistant. During that time I profited from the tax treaty. However, in 2008 I started my PhD and I'm still teaching 2 classes during the year and I work as an instructor in summer. I also got a fellow. Now my questions is if I should still fill out the form for tax benefits, because I'm planing on staying for at least 2 more years? Could they ask me after 5 years to pay federal taxes for 2006-2011? Technically am I considered as a student or teacher?

I'm a NRA and my homeland is Germany.I entered the United States in 2006 and I did my Masters but also taught for 2 years as a teaching assistant. During that time I profited from the tax treaty. However, in 2008 I started my PhD and I'm still teaching 2 classes during the year and I work as an instructor in summer. I also got a fellow. Now my questions is if I should still claim for tax benefits? Could they ask me to pay back the federal tax from 2006-2011? I'm planing on staying in the US for at least 2 more years...
Also am I considered as a teacher or student? Also I received my fellow and paid already 14% federal tax and also state tax. Is there any possibility to fill a form and get these taxes back? Could anyone explain to me, what changes have been made in the tax treaty between Germany and the US in 2007, that might effect me?

You were eligible for the tax benefits as a student, which includes a $9,000 treaty exemption. However, that exemption expired after FOUR years, which, for you, was 31 December 2009.

However, you are NOT eligible for the total tax exemption allowed to teachers, because you were a student immediately before the tour as a teacher.

You can find an explanation of these facts under Article XI at the link below:

http://www.irs.gov/pub/irs-trty/germanyte07.pdf