My wife and I separated in September 2019. We did not divorce or legally separate as of 12/31/20 and are now I am preparing for my 2020 Federal Tax return. We did not live together for even 1 minute in 2020. I plan on filing Single for 2020. I cannot find anything that states that I cannot file Single. Can you provide any insight into the IRS rules governing Filing status for my situation?
If you have not received a final divorce decree and are still married you CANNOT file single. Your choice is to file a joint return if both spouses agree, file married filing separately, or file Head of Household if you have your child, foster child or step child that lived with you more than half the year.
See IRS Pub 17 "Marital Status" page 21
Note: that "Legal Separation" requires a court order and separate maintenance which is the same as alimony in a divorce.
In general, your filing status depends on whether you are considered unmarried or married.
Unmarried persons. You are considered unmarried for the whole year if, on the last day of your tax year, you are either:
Legally separated from your spouse under a divorce or separate maintenance decree.
State law governs whether you are married or legally separated under a divorce or separate maintenance decree.
Divorced persons. If you are divorced under a final decree by the last day of the year, you are considered unmarried for the whole year.