Thanks for reply! Sorry I should put all my backgrounds XD
I'm partially on F-1 OPT in 2020 then I become H-1B on October. Thus I didn't pay FICA before October and the payroll forgot that I do need to pay them after my status become H-1B. They corrected it by a "lump sum" in my 2021 pay check.
For the questions:
1. I filled extension and I use First Year Choice now, to be Dual Status in 2020. This way I pass substantial presence test June this year. https://www.irs.gov/individuals/international-taxpayers/substantial-presence-test
2. I'm not sure if it's an "error". https://www.irs.gov/forms-pubs/about-form-w-2-c
3. I'm not sure if I follow. To my understanding my employer should withhold FICA after my status changed to H-1B but they didn't, and instead withhold the missing amount in my 2021 pay check.
For other background information:
1. 2020 it's my first year working full-time, in F-1 OPT and H-1B.
2. I filled 1040NR in 2019.
3. I'm not married.
