Overview

Filing taxes as a nonresident alien or a part‑year resident is fundamentally about two questions: (1) Are you treated as a U.S. resident for tax purposes during any portion of the year? and (2) what income is considered U.S.‑source or taxable while you were a resident? The answers determine which return you file (Form 1040 or Form 1040‑NR), what income you must report, available deductions and credits, and the deadlines that apply.

This guide explains the residency tests, who must file, common filing scenarios (including dual‑status and split‑year situations), deadlines and extensions, common mistakes, and practical next steps. Authoritative guidance comes from IRS Publication 519 (U.S. Tax Guide for Aliens) and the Instructions for Form 1040‑NR; consult those sources and a tax professional for complex cases.

(Author’s note: In my 15 years advising international clients, the two most frequent issues I see are misidentifying residency status and failing to apply tax treaties correctly. Both errors can lead to unexpected tax bills or missed treaty benefits.)

How residency is determined

Two primary federal tests establish tax residency:

  • Green card test: You are a U.S. resident for tax purposes for any year in which you are a lawful permanent resident at any time (you hold a green card) (see IRS Publication 519).
  • Substantial presence test: You meet this test if you are physically present in the U.S. for 31 days in the current year and 183 days total using the IRS weighted formula over the current and two prior years (current year days + 1/3 of prior year + 1/6 of second prior year). If you meet the substantial presence test, you are generally treated as a U.S. resident for that year.

If you neither hold a green card nor meet the substantial presence test, you are generally a nonresident alien for tax purposes.

Special categories and exceptions exist (students and teachers on certain visas, closer connection exceptions, etc.), and those exceptions are explained in IRS Publication 519.

Who must file: general rules

  • Nonresident aliens: File Form 1040‑NR if you had U.S.‑source income that is subject to U.S. tax and does not have all tax withheld at source (or you want a refund). Examples include wages earned in the U.S., effectively connected income from a U.S. trade or business, and certain investment or rental income. See the Instructions for Form 1040‑NR for details.

  • Part‑year residents (split‑year): If you became or ceased to be a resident partway through the year, you generally file Form 1040 as a resident for the portion of the year you were a U.S. resident and report worldwide income for that resident period. For the nonresident period, you report only U.S.‑source income. These split‑year or dual‑status rules are detailed in IRS Publication 519 and are discussed in more depth in our guide to Tax Filing Considerations for Split‑Year Residency.

  • Dual‑status aliens: If you were a dual‑status alien (part nonresident, part resident), you may need to file a dual‑status return. The filing rules and limitations on exemptions and credits differ from full‑year returns; see IRS guidance and consider professional help.

Which forms to use and what to report

  • Form 1040 (U.S. Individual Income Tax Return): Used by U.S. residents and part‑year residents for the resident portion of the year. As a resident, you report worldwide income earned during your residency period.

  • Form 1040‑NR (U.S. Nonresident Alien Income Tax Return): Used by nonresident aliens to report U.S.‑source income and certain effectively connected income. For details about which situations require Form 1040‑NR, see our primer When to Use Form 1040‑NR for Nonresident Aliens and the official Instructions for Form 1040‑NR.

  • Other forms commonly involved:

  • Form W‑8BEN or W‑8BEN‑E (to claim treaty benefits for withholding on investment income)

  • Form 8233 (to claim a personal services tax treaty withholding exemption in certain cases)

  • Form 8843 (nonresident students/trainees exempted from substantial presence)

  • Schedule NR or state nonresident forms (many states have separate nonresident filing rules).

We also discuss moving from nonresident to resident status and the tax impact in more detail in our article on How Tax Treaties Affect International Income and Withholding.

Filing deadlines and payment timing (practical summary)

  • Residents and part‑year residents who file Form 1040: The regular filing deadline is generally April 15 (or the next business day if April 15 falls on a weekend or holiday). If you need more time to file, you can request an extension of time to file (Form 4868) which generally extends filing to October 15 but does not extend the time to pay tax owed.

  • Nonresident aliens filing Form 1040‑NR: Filing deadlines depend on whether you received wages subject to U.S. income tax withholding. If you received wages subject to withholding, the due date is generally the same as for residents (April). If you did not receive wages subject to withholding, a later due date (often June 15) may apply. Specific deadlines are in the Instructions for Form 1040‑NR and can vary slightly by year; confirm current due dates on the IRS site.

  • Payments: Even if you are granted extra time to file, you should estimate and pay any tax owed by the regular due date to avoid interest and penalties. Nonresidents with U.S. income may also need to make estimated tax payments (see Form 1040‑ES or Form 1040‑ES (NR) guidance).

Note: Dates can change and some relief rules apply in disaster or coronavirus years. Always confirm current deadlines on IRS.gov or with a tax advisor.

Common mistakes and traps

  • Misclassifying residency: Treating a year as nonresident when you meet the substantial presence test is a frequent mistake. Run the day‑count test carefully and check exceptions for students, teachers, and medical conditions (Publication 519).

  • Using the wrong form: Filing Form 1040 instead of Form 1040‑NR (or vice versa) can lead to incorrect reporting of worldwide income or missed treaty benefits.

  • Overlooking treaty benefits: Income that appears taxable may be exempt or taxed at a reduced rate under a tax treaty. To claim treaty benefits you often must attach statements and use the correct withholding forms (W‑8BEN, Form 8233). Our piece on treaties explains common scenarios.

  • State filing obligations: States have their own residency rules. Being a nonresident for federal tax purposes does not automatically exempt you from a state return if you have state‑source income or maintained a domicile.

  • Deadline confusion: Different deadlines for nonresidents, residents, and extension rules cause costly late‑filing penalties if missed.

Practical checklist before you file

  1. Determine your residency status for the tax year (green card, substantial presence test, exceptions).
  2. Identify all U.S.‑source and worldwide income for the relevant periods.
  3. Decide which form(s) you must file (Form 1040, Form 1040‑NR, or a dual‑status return).
  4. Check applicable tax treaties and prepare supporting forms (W‑8BEN, Form 8233 if applicable).
  5. Gather documentation: passport/visa dates, Form W‑2, Forms 1099, rental records, bank statements, proof of days in/out of the U.S.
  6. Confirm federal due dates and any state filing requirements; estimate and pay taxes by the due date to reduce interest and penalties.
  7. Consider professional help for complex items (dual‑status returns, treaty claims, foreign tax credits).

Real‑world example (illustrative)

A client moved from Canada to the U.S. on June 1 and became a resident on August 15 after passing the substantial presence test. For the period January 1–August 14 they were a nonresident and only U.S.‑source wages earned in that period were taxable to the U.S. From August 15 through December 31 they were a resident and had to report worldwide income earned during the resident portion. We prepared a resident return for the year and attached required statements to claim a foreign tax credit for taxes paid to Canada during the resident period. This split‑year approach avoided double taxation and complied with IRS guidance.

When to get professional help

Seek a tax professional if you have:

  • Dual‑status questions or split‑year residency
  • Treaty eligibility or reduced withholding claims
  • Mixed U.S. and foreign income, foreign tax credits, or complex foreign asset reporting
  • State filing questions tied to domicile or days present

In my practice, early documentation (travel logs, pay stubs, foreign tax statements) makes these situations straightforward to resolve. Waiting until the filing season often increases the cost and risk of errors.

Resources and authoritative guidance

  • IRS Publication 519, U.S. Tax Guide for Aliens (residency tests, dual‑status rules, treaty information)
  • Instructions for Form 1040‑NR (filing requirements for nonresident aliens)
  • IRS Form instructions: Form 1040, Form 4868, Form 8233, Form W‑8BEN

Further reading on FinHelp:

Final notes and disclaimer

This article summarizes common filing rules for nonresident and part‑year residents based on IRS guidance current as of 2025. It is educational and does not replace personalized tax advice. For specific situations, treaty questions, or complex international income, consult a qualified tax advisor or the IRS Publications cited above.