If you are an alien (not a U.S. citizen), you are considered a nonresident alien unless you meet one of two tests. You are a resident alien of the United States for tax purposes if you meet either the green card test or the substantial presence test for the calendar year (January 1-December 31).
You can be both a nonresident alien and a resident alien during the same tax year. This usually occurs in the year you arrive or depart from the United States. If so, you may elect to be treated as a Dual Status Alien for this taxable year and a Resident Alien for the next taxable year if you meet certain tests. (Refer to section "Dual-Status Aliens" – "First Year Choice" in Publication 519, U.S. Tax Guide for Aliens.)