I am currently on a tax treaty exemption Belgium/USA and on a J-1 visa. The time for the tax exemption is 2 years. What I want to know is, if I decide to stay on for another year following the two years, will I have to back-pay tax or will I just have to pay the tax on the third year?

I would have to review your case in detail to be sure, but the normal rule is that you must pay the two-year back tax if you exceed the two-year period in the U.S.

The Belgium-US treaty does not require retroactive tax.

I yield to YYE's greater expertise in this area!