For tax purposes, if you're not a US Citizen or U.S. Resident, you're a non-resident alien. If you do not meet the tests for being a Resident Alien, you are considered a non-resident Alien. You are considered a Resident Alien if you are a green card holder or meet the substantial presence test. To meet the substantial presence test, you must have been physically present in the country:
For 183 days or more in the tax year; or
31 days in the tax year and 183 days during the prior 3 year period that includes the tax year (count all days you were present in USA), year prior to the tax year (counting one third of the days you were present in the US) and two years prior to the tax year (counting one-sixth of the days you were present in the US).
If you did not meet the substantial presence test for the tax year, you could still make a First Year Choice (full details on Page 7 of IRS Publication 519, U.S. Tax Guide for Aliens) if you meet the substantial presence test the years after your tax year.
If you choose to make this choice, you cannot file your returns for your tax year until you meet the substantial presence test the following year. You would need to file an extension and wait until you meet the test before filing your tax returns.