Do I need to file a U.S. tax return if I'm a non-resident?

Yes, if you are a non-resident alien engaged in a trade or business in the United States, you must file Form 1040NR. This requirement applies even if your income is less than the personal exemption amount, your income is exempt under a tax treaty, or you have no U.S.-sourced income. The filing deadline is April 15 if you received wages subject to U.S. income tax withholding, or June 15 if you did not receive wages.
What's the difference between ECI and FDAP income, and why does it matter?

Effectively Connected Income (ECI) is income earned from a trade or business you conduct within the United States, such as wages or salary from a U.S. employer. ECI is taxed at the same progressive rates that apply to U.S. citizens and residents, and you can claim certain deductions against this income.
FDAP Income (Fixed, Determinable, Annual, or Periodical) is generally passive income like interest, dividends, and royalties. FDAP income is taxed at a flat 30% rate with no deductions allowed, unless a tax treaty provides a lower rate. Understanding this distinction is crucial because it determines how your income is taxed and what deductions you can claim.
Can I claim the standard deduction as a non-resident?

No, with very few exceptions (related to students and business apprentices from India), non-residents cannot claim the standard deduction. You must itemize deductions on Schedule A of Form 1040NR. However, you can only claim deductions to the extent they are connected with your Effectively Connected Income (ECI). Common itemized deductions include state and local taxes, charitable contributions to qualified U.S. charities, and casualty losses.
How can tax treaties help reduce my U.S. tax liability?

Tax treaties between the United States and your home country can significantly reduce your tax burden by providing reduced tax rates or complete exemptions on certain types of income. For example, many treaties reduce the standard 30% tax on dividends to 15%, 5%, or even 0%. Some treaties also offer temporary tax exemptions for students or researchers. To claim treaty benefits beyond simple withholding rate reductions, you may need to file Form 8833 (Treaty-Based Return Position Disclosure) with your return.
Do I need to file state tax returns in addition to my federal Form 1040NR?

Yes, if you have income sourced to a state that has an income tax, you likely need to file a state tax return. This is required even if you file a federal return. Each state has its own forms, rules, and filing thresholds. For example, if you earned income in California, you would file Form 540NR, while income in New York requires Form IT-203. Nine states currently have no state income tax: Alaska, Florida, Nevada, New Hampshire, South Dakota, Tennessee, Texas, Washington, and Wyoming. State tax compliance adds complexity to your situation, making it an area where professional assistance can be particularly valuable.