Most years, a US filer is cleanly either a resident alien or a nonresident alien for the whole year. The year you actually move — arriving to take up residency, or departing for good — is often the exception: you can be a nonresident for part of the year and a resident for the rest, which the IRS calls dual-status. It changes how the return is structured, not just what boxes get checked.
What actually changes
For the part of the year you're a resident, you're taxed on worldwide income, the same as any US resident. For the part of the year you're a nonresident, you're taxed only on US-source income. A dual-status year isn't one Form 1040 — it's a combination filing (a 1040 paired with a 1040-NR statement, or the reverse, depending on which status applies at year-end), and several standard resident-year conveniences, like claiming the standard deduction or filing jointly with a spouse, generally aren't available in a dual-status year without a separate election.
The First-Year Choice election
If you arrive mid-year and wouldn't otherwise pass the Substantial Presence Test until the following year, the First-Year Choice election can let you be treated as a dual-status resident starting partway through the arrival year instead of waiting until you clearly qualify — but only if you're present for at least 31 consecutive days in the current year and present for at least 75% of the days from that 31-day period through the end of the year. It's an election you make, not something that applies automatically.
This is exactly the kind of transition-year situation where the Substantial Presence Test calculator on this site's DTAA & Tax Residency page is most useful — not just for a yes/no answer, but for pinning down the specific day-count that determines whether a dual-status year applies to you at all, and where your residency starting date falls within it.