"Resident" for tax purposes and "resident" for immigration purposes are different things. You can be a non-immigrant on a temporary visa and still be a resident alien for tax. That determines which form you file and what income the US can tax.

The two tests

A non-citizen is a nonresident alien unless they meet the green card test or the substantial presence test.

The green card test. You were a lawful permanent resident at any time during the calendar year — you held Form I-551. Residency begins on the first day you are present in the US as an LPR, and continues until the status is voluntarily renounced in writing to USCIS, administratively terminated by USCIS, or judicially terminated. Leaving the country does not end it.

The substantial presence test. You were present in the US for:

• at least 31 days in the current year, and
• 183 days across a three-year period, counting:
  • every day in the current year
  • 1/3 of the days in the first preceding year
  • 1/6 of the days in the second preceding year

So 120 days a year for three years is 120 + 40 + 20 = 180. Under the line. One extra week in the current year puts you over it.

Days that do not count

Several categories of day are excluded from the count:

• Commuting from Canada or Mexico on a regular basis
• Transit of under 24 hours between two points outside the US
• Crew of a foreign vessel
• Days you could not leave because of a medical condition that arose in the US
• Days as an exempt individual

Exempt individuals are the big one, and it catches a lot of people by surprise in a good way:

• Foreign-government-related individuals on an A or G visa (not A-3 or G-5)
• Teachers or trainees on a J or Q visa
• Students on an F, J, M or Q visa
• Professional athletes at a charitable event

Exempt individuals file Form 8843, even if they have no income and file no return. A student on an F-1 who does not file the 8843 is failing a filing obligation, quietly, every year.

A closer connection exception also exists for those who meet the presence test but maintain a tax home and closer connection to another country.

Which form

Status	Form	Taxed on
Resident alien (green card or substantial presence)	Form 1040	Worldwide income
Nonresident alien	Form 1040-NR	US-source income only
Arrival or departure year	Dual-status return	Split — see below
Foreign-owned single-member LLC	Pro forma 1120 + Form 5472	Information return

The worldwide-versus-US-source distinction is the whole game. A resident alien reports income from every country. A nonresident reports only what the US sourced.

Dual-status years

In the year you arrive or leave, you may be a nonresident for part of the year and a resident for the rest. That produces a dual-status return: a 1040 for the resident period with a 1040-NR as a statement for the non-resident period, or the reverse.

Dual-status returns carry restrictions — no standard deduction, limits on filing status — and a first-year choice election exists that can sometimes produce a better outcome. It is a genuinely technical area.

1040-NR deadlines

Different from the 1040, and this catches people:

• 15 April if you were an employee or self-employed and received US wages subject to withholding
• 15 June if you were not an employee and had no place of business in the US

There is a further trap: a 1040-NR filed more than 16 months after the due date may lose the right to claim deductions and credits, leaving you taxed on gross rather than net income.

The business forms

If you own a US entity, the entity has its own filing. For a foreign-owned single-member LLC that is a pro forma Form 1120 with Form 5472 attached — an information return with a $25,000 starting penalty for non-filing. See how to file US business taxes as a non-resident owner.