Resident and Non-Resident aliens of the United States are taxed in the same manner as are U.S. citizens. In addition, you are required to complete W-4 forms to avoid under-withholding by:
Non-Resident aliens of Canada, Mexico, and Korea may claim more than one allowance. For exceptions to these rules, visit the IRS.
See IRS Pub.519 US Tax Guide for Resident aliens.
See IRS Pub.515 Tax Withholding, Non-Resident aliens.
See IRS Notice 1392 Supplemental Form W-4 Instructions for Non-Resident aliens.