You have a significant problem, and you may need to see an attorney.
Kansas is one of the states that recognizes common law marriage. A common law marriage is a legally binding marriage even without a marriage license or ceremony. If you have a valid common law marriage, then you are married for all purposes and in any state you move to in the future because once you are married in one state, you are married in all states. The only way to end a common law marriage is with a real divorce with real attorneys at a judge. You can’t turn common law marriage on and off like a light switch.
I do not have the specific laws of Kansas at hand, but to have valid valid common law marriage generally requires four things.
1. you are not married to anyone else.
2. you live together as if you are married.
3. you act in most other ways as if you are married, such as sharing finances or sharing a bank account.
4. you tell at least one other person or make it public knowledge that you are married.
Now, even if you have not told anyone else that you are married, the fact that you filed a tax return last year as if you are married may have satisfied that public declaration requirement. In which case you are legally married, now and forever, in sickness and in health, etc. until you see a judge about a divorce.
So now turning to your tax situation.
If you do not have a valid common law marriage, you cannot file as married. You must go back and correct your 2019 tax returns to file as single. If you had the child in 2019, only one of you can claim the child as a dependent. That person might be allowed to file as head of household which has a lower tax rate and better deductions. For 2020, you must file as single. Only one of you can claim the child as a dependent. The person who claims the child as a dependent might be able to file as head of household. The other taxpayer who does not claim the child as a dependent must file as single.
If you determine that you do have a valid common law marriage, then you can only file as married. You would file two returns as married filing separately, with each spouse listing their individual income on their return; or, you would file as married filing jointly again, and list all your combined income and dependents on one return.
You simply can’t file as married filing jointly last year, and single this year. Either you were married last year, in which case you are also married this year, or you weren’t married last year and must correct last year‘s tax returns.
