很重要的 First Year of Residency
1. Nonresident Alien or Resident Alien?
6. Dual-Status Tax Year
数完符合条件的天数，看publication 519，page 5的流程图。如果我是上半年来的，那么当年〉183天，那么“You are a resident alien for U.S. tax purposes."
看流程图下面的小字注解1 - If this is your first year or last year fo residency, you may have a dual status for the year.
See Dual-Status Aliens in chapter 1.
什么是dual status呢？Publication 519, page 8 说了：
"You can be both a nonresident alien and a resident alien during the same tax year. This usually occurs in the year you arrive in or depart from the United States."
黑体字 First Year of Residency 的下面很重要：
If you are a U.S. resident for the calendar year, but you were not a U.S. resident at any time during the preceding calendar year, you are a U.S. resident only for the part of the calendar year that begins on the residency starting date.
You are a nonresident alien for the part fo the year before that date.
这说明"resident alien" 与 full year resident 不是自动等同的。很多人想到“resident”就认为是full year，是不对的。
The term "resident" is loosely used by many people, in many places, including some IRS publications.