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).
For more information see this link
Determining Alien Tax Status
https://www.irs.gov/individuals/international-taxpayers/determining-alien-tax-status
