So, we got divorced on August 2019. We filed joined married last year. This year we filed single. Everything was good until Oregon kicker part. Let say our income was 100000 (my wife 25000) and AGI is 80,000 after all deduct. 2020 tax return want to know how much my contribute to 80,000 AGI so that they can give me the the kicker? Do I have to refile my 2018 tax as single to get the correct AGI?
No, you would not refile your 2018 return as single as you were still married at the time.
You will need to determine if your ex-spouse has claimed any of the credit. Essentially, you will need to make sure that both of you combined do not report more than your 2018 tax liability amount that is used to calculate the credit.
The amount of your credit would be the amount of your 2018 tax liability before any credits multiplied by 16%. (Or Form OR-40, LIne 22 x 16%).
This amount will be reported on your 2019 Oregon tax return.
For example, if your 2018 tax liability was $1,000, then the maximum credit would be $160. Between both of you, the total claimed on your individual returns should not exceed the $160. You will need to discuss it to ensure you don't take more of the credit than you are allowed to claim.