OPT期間Tax的問題: Resident Alien vs. Nonresident Alien - 美國

有個關於OPT期間Tax的問題想要請教板上對此議題了解的人

我以學生身分來美國兩年半了,
目前是 under OPT status, 就業中
(OPT Jan.2009 到期; H1b Application前天剛寄出)

一直以來總是聽說 在OPT期間, 不須繳交Social security & Medicare Taxes
所以本來我明天打算跟老闆說我現在是OPT
所以不用預扣SS & Medicare tax


但是 我剛剛查了 Publication 519, Ch8 U.S. Tax Guide for Aliens 之後,
(Under "Social Security and Medicare Taxes" Section)
我現在非常困惑....

它提到 "Generally, services performed by you as a "nonresident alien"...
.... under subparagraph (F), (J), (M), or (Q) [visa type]......
...This means that there will be no withholding of social security
or Medicare taxes from the pay you receive for these services.
These types of services are very limited, and generally include only
on-campus work, practical training, and economic hardship employment"
(因為原文太長...所以我只highlight了重點)

但是下一段它又說
Social security and Medicare taxes will be withheld from your pay
for these services if you are considered a resident alien as
discussed in chapter 1, even though your nonimmigrant classification
(“F,” “J,” “M,” or “Q”) remains the same.

所以簡單的講
如果持有F-1 & OPT 中 + nonresident alien = no withholding of SS & Medicare tax
如果持有F-1 & OPT 中 + resident alien = withholding of SS & Medicare tax

但是問題在於 只要過去三年內在美國待滿183天,
便通過 Substantial Presence Test,
可稱之為 resident alien..

但是,話又說回來...板上大部分能夠申請opt的人,
應該都是在過去三年內在美國待滿183天吧~
所以應該是resident alien, 那就應該會被withhold SS & Medicare tax...?!

但是大家又都說不用繳...
難道是先withhold,之後再退稅??
還是我把 nonresident alien & resident alien的定義搞錯了?!?!

請了解的人稍微解釋一下可以嗎??
謝謝!!
(方便的話請順便回我信箱)
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