I am a teacher on j1 visa in South Carolina working in my final third year
I was exempted from tax state and federal for 2 yrs ,now they say tax will be deducted from jan 2008
My question is will the tax be from jan 2008 or it will be retrospective from the day of my presence in US ,kindly guide
Anjum

It depends upon the tax treaty between your country and the U.S. Normally, the exemption for the 2 years is lost.
Read in IRS Publication 901: Tax Treaties; Professors, Teachers, and Researchers.
Internal Revenue Service