Nonresident aliens are non-citizens of the U.S. They are exempt from a Green Card. Teachers, students, foreign nationals (for tax purposes), and those seeking medical services are common nonresident aliens. Nonresident aliens pay taxes on a trade or business related to the U.S. Resident aliens (qualified persons under the substantial presence test) are those who have been in the U.S. for 31 days or resided in the U.S. for more than 183 days within a 3 year period.