You probably can't file single, but it depends on the laws of your state. You are "considered unmarried" if you are divorced or legally separated under a qualifying court order. The problem is that most states don't have a court-ordered separation that meets the IRS definition. For example, New York and New Jersey do not. Even if you file a separation agreement with the court, that is not enough of a "court order" to qualify to file as single. (Since it costs the same in time and money to get a court ordered separation as a divorce, no one ever does it as a practical matter. You do a mediated separation and then a final divorce order.)
So probably you have to file as married filing separately. But if you want to be sure, check with a tax attorney in your state.
If you have a qualifying dependent, and cared for that person in your home, and paid more than half the cost of keeping up the home, and were separated since at least July 1, you can file as head of household.
