Hi pk,
Thank you very much for your answer! I got the SPT wrong because I was confused between calendar year and tax year. We file tax for the year before. Although in year 6 I become a resident for tax purposes, I was a non-resident in year 5.
This would be helpful for future readers.
I will amend using 1040-X, include 1040-NR (which my school helps by giving us discount code for Sprintax) and pay the tax due. Thanks again!
