I have been an international student in the US on f-1 visa since 2006. On 2011,( my 6th year in the US) I did my OPT from January to August and on August I started a masters degree program again on F-1. What form do I have to file for 2011?

You get only ONE five-year period on F-1 during which time you are considered to be a non-resident alien. Even though you are on a valid F-1 visa, you are a resident alien for tax purposes and should file Form 1040 or 1040A.

The GOOD NEWS is that you can claim the education credits for the tuition you are paying for the master's degree you seek on your 2011 tax return.

The BAD NEWS is that, during your OPT, you WERE LIABLE for FICA (Social Security and Medicare) taxes, and have a legal obligation to tell your employer about that fact. You should do so IN WRITING (email is good for that).

If you want my professional help, double-click on my title above and scroll down to my profile/signature. You will find my email address and websites there.

For 2011 you are resident.