If you're required to file a federal return for 2021, you can use Form 1040 (U.S. Individual Income Tax Return) if any of these are true:
- You're a US citizen
- You're a resident of the US or Puerto Rico
- You have a Permanent Resident Card ("green card")
- You're a nonresident alien married to a US citizen/resident alien with whom you'll file a joint return
- You meet the substantial presence test, which means you were physically present in the US for at least 31 days in 2021 and at least 183 days in 2019 through 2021, according to this formula:
2021 days + (2020 days)/3 + (2019 days)/6 >= 183
Example: Jo works 3 months (90 days) every summer as a camp counselor in the US. They meet the first part of the test (at least 31 days in 2021), but fails the 183-day test:
31 + 90/3 + 90/6 = 31+30+15 = 76 days
Because Jo didn't meet the substantial presence test, they're considered a nonresident alien for federal tax purposes and can't file a 1040. Instead, they must file a 1040-NR.
In addition to those who don't pass the substantial presence test, nonresident aliens also include:
- Foreign nationals
- Anybody who isn't a US citizen, US or Puerto Rico resident alien, or doesn't hold a green card
- A person who has income from or participates in a US-sourced trade or business
Unlike the 1040, there's no minimum income threshold for filing a 1040-NR. In other words, nonresident aliens with US income, regardless of amount, are required to file a 1040-NR.