When you say "SO" you mean someone to whom you are not legally married? You are not allowed to file a joint return unless you are legally married, so it is SO who has to file an amended return--not you. SO will have to amend once his/her return is fully processed and he has received his refund. He will have to prepare a 1040X in order to do that.
You can file as single or HOH (if you are the one claiming the children). Since SO has already put your Social Security number into the system for 2017, you will not be able to e-file now. You will have to print, sign and mail your tax return instead.
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 received the return.
**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.**
