What is the green-card test for tax residency?

The green-card test is the simplest path to U.S. tax residency: if you're a lawful permanent resident (you hold a green card) at any time during the calendar year, you're treated as a resident alien for tax purposes for that year. It doesn't matter whether you actually spent much time in the U.S.—status, not days, is what counts here, unlike the Substantial Presence Test. As a resident alien you report and pay tax on worldwide income on Form 1040. Your residency starting date is generally the first day you're present in the U.S. as a green-card holder. This status continues until your permanent-resident status is officially abandoned or revoked, even if you live abroad. Green-card holders moving overseas often keep U.S. filing obligations for years, so don't assume leaving the country ends your duty to file.