@Blythe Matthews As already explained in this thread, if someone else claimed a dependent that YOU can claim on your own tax return, you have to file your own return claiming that child by MAIL. You cannot e-file. If you did not file or claim the child for those past several years, you can still do so. Those returns must be filed by mail. They cannot be e-filed.
You say your grandchild was claimed by the child's mother. Who did the child live with? The IRS cares about physical custody and who the child actually spent the most nights with. If you are the one the child lived with, then you can claim the child on your own tax returns. File or amend returns as far back as 2020 if necessary to claim the grandchild on your own tax return.
When you mail a tax return, you need to attach any documents showing tax withheld, such as your W-2’s or any 1099’s. Use a mailing service that will track it, such as UPS or certified mail so you will know the IRS/state received the return.
Federal and state returns must be in separate envelopes and they are mailed to different addresses. Read the mailing instructions that print with your tax return carefully so you mail them to the right addresses.
IRS interview to help determine who can be claimed:
https://www.irs.gov/help/ita/who-can-i-claim-as-a-dependent
**Disclaimer: Every effort has been made to offer the most correct information possible. The poster disclaims any legal responsibility for the accuracy of the information that is contained in this post.**
