Your visa status can help determine your filing status because certain visa categories must be treated as nonresident aliens for a specific period of time. (hrblock.com)
Fast forward now (2014), I realized that I was in US for half a year and should have filed as nonresident alien instead of resident alien. (money.stackexchange.com)
A flat tax of 30 percent was imposed on U.S. source capital gains in the hands of nonresident alien individuals physically present in the United States for 183 days or more during the taxable year. (money.stackexchange.com)