What's the difference between the green-card test and the substantial presence test for deciding tax residency?

The IRS uses two independent tests, and meeting either makes you a resident alien. The green-card test is simple status: if you're a lawful permanent resident at any point in the year, you're a resident from your first day of presence with that status. The substantial presence test is a day-count: weighted U.S. days over three years reaching 183, with the 31-day minimum. Visa holders like H-1B and L-1 who don't yet have green cards rely on the substantial presence test, while green-card holders qualify automatically regardless of days, even while living abroad. You can pass one, both, or neither in a given year. IRS Publication 519 explains both tests at irs.gov.