Yes, you I need to mail the form to IRS along with a 1040X, and it will not mess up your tax filing record since you filed the 2019 tax online but do the amendment by mail.
No, you do not have to wait until IRS accepts the amendment to file my 2020 return so that I can use the correct basis. You can file your 2020 as if you have a form 8606 for 2019.
Yes, you can add a form 8606 for a previous tax year but you can't add it after it was filed or amended. You would have to amend (again) if you already amended.
To trigger the 8606 in TurboTax 2019 please follow these steps:
- Log into TurboTax 2019. Here is the link.
- Inside TurboTax, search for this exact phrase, including the comma: 8606, nondeductible ira contributions
- Select the Jump to link in the search results.
- Proceed through the IRA section, answering questions as you go.
- When you reach the screen Any nondeductible contributions to your IRA? answer Yes, then continue. We'll generate and fill out the 8606 behind the scenes.
Here is a TurboTax article with more information about form 8606.
