Hello, @pk. Thank you for your message! The answers to your questions are below:
(a) I entered US on J1 visa in January 2023 as a Research Scholar.
(b) I am from Brazil (as far as I know, there is no tax treaty)
(c) Yes, I never left the US after I first came.
So, for the calendar year of 2025 I will be treated as a resident for tax purposes. But I am not sure about the calendar year of 2026, which I will be here only for a few months. I have read about dual tax status, but not sure how or if that would apply depending on when I will leave (January or end of March).
