Nonresidents of the U.S. are generally not subject to any U.S. federal income taxes unless they have U.S. source income. U.S. source FDAP income paid to nonresidents is subject to withholding taxes at source.
If a non-U.S. citizen is temporarily living and working inside the USA, they are generally considered a U.S. tax resident. When the person leaves the USA, they revert back to being a nonresident for federal tax purposes.
A nonresident non-citizen of the USA may be required to file a Form 1040-NR if they have U.S. source income. This article and video discuss those potential filing obligations.
Certain U.S. tax residents that leave the country may be required to file an expatriation statement with the IRS. This article and video discuss the basics of Form 8854.
