If you were separated since July 1, 2023, and you provide care in your home for a qualifying child dependent, and you pay more than half the cost of maintaining your home, you can file as head of household, that's better than single.
If you are legally married, don't qualify for head of household, and want to be considered "unmarried" so you can file as single, the IRS says you must be "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."
The short answer is that you must seek the advice of a local tax professional. We are not attorneys and can't judge whether your separation meets IRS standards.
The longer answer is that, it is very difficult for a legal separation to actually qualify you to file as single. I read a Tax Court case from New York during my own divorce. The ruling essentially was that separated spouse can't file as single. The Tax Court noted that voluntary separations that allow the legal possibility of the parties to reconcile don't meet the standard for being single, even if they are under court supervision. New York does have a separation law on the books from decades ago (when divorce was more controversial and opposed by certain religions) that the parties must remain forever separate, may not interfere with one another, and are in all other ways divorced, except they could not remarry someone new. That type of separation would qualify, but for obvious reasons, no one in New York actually uses that kind of separation any more, so being separated in New York does not count as being "unmarried" so you can file single.
But, for a definitive opinion, you will have to talk to your own expert who knows your state laws and any relevant Tax Court cases.
