An alien is any individual who is not a U.S. citizen or U.S. national. A nonresident alien is an alien who has not passed the green card test or the substantial presence test.
If you are a nonresident alien at the end of the tax year, and your spouse is a resident alien, your spouse can choose to treat you as a U.S. resident alien for tax purposes and file Form 1040 using the filing status “Married Filing Jointly.”
Who Must File
You must file a return if you are a nonresident alien engaged or considered to be engaged in a trade or business in the United States during the year. However, if your only U.S. source income is wages in an amount less than the personal exemption amount (see Publication 501, Exemptions, Standard Deduction, and Filing Information), you are not required to file.
Even if you are not engaged in a trade or business in the United States, you must file a return if you have U.S. income on which the tax liability was not satisfied by the withholding of tax at the source.
You also must file an income tax return if you want to claim a refund of access withholding or want to claim the benefit of any deductions of credits (for example, if you have income from rental property that you choose to treat as income connected to a trade or business).
Tax Treatment of Nonresident Alien
If you are a nonresident alien engaged in a trade or business in the United States, you must pay U.S. tax on the amount of your effectively connected income, after allowable deductions, at the same rates that apply to U.S. citizens and residents. If you are not engaged in a trade or business, the payment of U.S. source income that is fixed, determinable, annual, or periodical is taxed at a flat 30 percent (or lower treaty rate) and no deductions are allowed against such income. You may earn both effectively connected income and fixed determinable, annual, or periodical income in the same year and they will be taxed accordingly.
See Alien Taxation – Certain Essential Concepts for a summary of some rules that apply to resident and nonresident aliens.