I just found out that I've been doing my parent's tax wrong.

My Dad gave up his green card 11 years ago. My mother retained her residency and became a citizen 2 years ago. I've been filing their tax jointly for last eleven years with their residence status being in California. My Dad co-owned a house with my mother with no mortgage. They also co-owned some share in an apartment complex that bring them income of less than 30,000 per year. My mom doesn't work. My Dad work abroad and has only spend less than one month in the last 10 years in the US as a visitor. So jointly, they haven't need to pay any tax at least last several years, they didn't get any tax return stimulus either in the last few years. So do I start fresh from next year or do I need to amend their tax for the last 11 years! Thanks for any advice!

Susan

Yes, you can file joint return as residents if one of the spouse is resident or U.S. citizen. On the return, both must report the worldwide income. On the joint return, your dad will be treated as resident.

You will need to amend if you did NOT claim your father's foreign income. Otherwise, no.