When does an immigrant become a "resident alien" for US tax purposes, and why does it matter?

You become a resident alien for tax purposes when you either hold a green card or meet the "substantial presence test", broadly, 31 days this year and 183 days counted over the current and two prior years (with prior years weighted). Most H-1B and L-1 workers meet it in their first year. F-1 students are usually exempt from counting days for their first five calendar years, so they stay nonresident longer. It matters a lot: residents are taxed on worldwide income and file the regular Form 1040, can claim the full standard deduction ($15,000 single / $30,000 married filing jointly in 2025), and report foreign accounts (FBAR/Form 8938). Nonresidents file Form 1040-NR and are taxed mainly on U.S. income. Know which bucket you're in before filing.