In the United States, with regard to federal income tax only, you need to know:whether the individual is a citizen, a non-citizen national, a lawful permanent resident, or some other category, and if in the u201csome other categoryu201d, how many days that person has been physically present in the United States over the past year, and possibly also the year preceding and the year before that as well;how much money that person earned through:wages on which Social Security taxes were collected,wages on which Social Security taxes were not collected,self-employment, andall other types of income not listed above (such as interest, dividends, royalties, capital gains, or gambling winnings);whether the person sold a home during the prior year;whether the person received payments from a retirement plan (including Social Security), and if so how much;how old the person is (specifically, whether or not under 19 and whether or not over 65);if the person is blind;whether the person is married, or not, and if not married, whether the reason for that was that the personu2019s spouse died during the past year;whether the person has children, and if so how many and whether or not they live with him or her;whether anyone else can claim the person as a tax dependent; andwhether the person received any advance payments of the health insurance marketplace premium credit during the prior year.And that doesnu2019t quite prall the information required to definitively determine whether a particular person is required to file a US federal income tax tax return; thereu2019s still a few edge cases that require information beyond the facts on this list to determine if a return is required. Also, not all of these facts will be required for every person.The instructions for Form 1040 (and its variants, Form 1040A and 1040EZ), for resident taxpayers, and Form 1040NR (and its variant, Form 1040NR-EZ), for nonresident taxpayers, have instructions that one can follow to determine if a filing is required, and if so what form must be used. Note also that if youu2019re not a US citizen, US non-citizen national, or US lawful permanent resident, youu2019ll also have to figure out whether youu2019re a resident taxpayer or a nonresident taxpayer in order to decide whether to file Form 1040 or Form 1040NR before you can figure out if you have to file at all. The IRS has a publication for that, too: Publication 519 (2022), U.S. Tax Guide for Aliens. (US citizens, US non-citizen nationals, and US lawful permanent residents are always u201cresident taxpayersu201d, even if they do not live in the United States.)The rules for when a state return is required are, of course, separate and distinct.