Hi, I am italian, I came in the US to work as post-doc (research scholar) in April 2009 with a J1 visa that will expire in 1 year. Probably I will stay in the US longer than 1 year, maybe 3 or 4 and I was told that I have a tax exemption for the first 2 years. If I will stay here longer than 2 years do I really have to pay the taxes I didn't pay during the first two years? I'm reading the "convention between US and Italy" (art.20) but there is not information about this. Can you help me? Thanks.

If there is no retroactive tax provison specifically and explicitly stated in the tax treaty, then there is NO retroactive tax provision.

That being the case, you start paying taxes after the two-year tax-exempt period provided for in the treaty.