If you as a nonresident are going to file a US tax return, you need to secure a taxpayer identification number also. That process has recently become somewhat cumbersome and difficult.
A nonresident working in the US may under the laws of the state in which he or she works also be required to file a state tax return.
Failure to file a tax return will result in the statute of limitations for later requiring a return or tax to never run out. Therefore, when in doubt you should as a nonresident always file a return.
The good news is that US nonresidents are not subject to tax on their interest income from banks, savings and loans and US treasury bills or on their capital gains from the sale of US stocks (so long as theses are investments and not connected with their US business).
Learn more about the US taxation of nonresidents at www.TaxMeLess.com