IRS definition of legally separated: "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."
If you are in fact legally separated with a court ordered separate maintenance decree then your filing status is single - not married, however very few states recognize legal separation and since a legal separation is so similar to a divorce and has almost no advantage, very few people legally separate.
If you were married as of the end of the tax year and not actually legally separated then you (and your spouse) can only file married filing separately unless you *both* agree (and sign) a joint return. When filing Married Separately then the spouses SSN is *required* to be on the separate tax return.
If you were actually divorced at the end of the tax year then your (and your ex) only filing status is single or possible head of household if a closely related relative was a dependent and lived with you.
Couple of points.
1. If you are still married, you can file as married filing jointly, if you both agree. This will usually result in lower taxes. If you did not agree, and if he filed jointly, then he filed fraudulently and you can file on your own and let the IRS investigate.
2. If you are still married, your other options are to file as married filing separately, or head of household if you provide care in your home for a custodial dependent child. You can rarely file as single if you are still married. The tax code is quoted above, but almost no one qualifies. For example, in NY state, separations are voluntary (even if under court supervision) and they are reversible (the couple could reconcile) so the IRS does not accept separation in NY state as allowing the spouses to file as "unmarried" or single. There are several tax court cases on this. If you want to file as single, be prepared to fight it out with the IRS.
If your spouse filed as married filing jointly, and you want to file as married filing separately, head of household or single, you will be blocked from e-filing. Print your return and mail it to the IRS and let them investigate. At some point you will need to be prepared to swear that he filed jointly without your permission. And if you are trying to file as single, you will likely have to defend that in court.
If your spouse filed as married filing separately, he is required to use your SSN. This is so the IRS can double check his return against yours. This will not block you from e-filing as head of household or married filing separately, because these are normal situations. But it will block you from e-filing as single. If you really think you qualify to file as single, print your return and mail it in. Otherwise, change your status to married filing separately.
If your spouse filed as married filing separately, he might also have claimed you as a dependent. He can only legally claim you as a dependent if you lived together all of 2016 and you had less than $4050 of your own income. If he did claim you as a dependent, you need to print your return and file by mail.
And finally, if your spouse filed as married filing separately and he itemized his deductions, then you must also itemize, even if you don't have any deductions. This is one big disadvantage of married filing separately but it is the law and there isn't any way around it unless you qualify to file as head of household. You might be getting a rejection if you aren't itemizing. You either need to itemize, or print your return and mail it in, but the IRS might bill you for the extra tax you may owe.
I recieved a letter back in August 2021 from IRS stating that I filed joint but i never did. I know my ex was the only one who knew and had access to my personal information and we were not married. He didn't file with personal taxes, so I believe he was able to hide it with his business. I'm investigating this situation but have not been successful at figure out who what and when. IRS wouldn't send letters unless it's something serious right?