A2A.IRS Publication 501 (2023), Exemptions, Standard Deduction, and Filing Information covers the conditions for determining filing status.A couple is considered married if they meet any one of the following tests on the last day of the tax year:married and living togetherliving together in a common-law marriage under state lawmarried and living apart but not legally separatedseparated under an interlocutory (not final) decree of divorceA couple is considered unmarried if they are divorced or legally separated under state law.A spouse cannot use the single filing status if considered married. A spouse that is still considered married can only use the married filing jointly or married filing separately status (in certain cases, a spouse may be able to use the head of household status when the spouses have not lived together at any time in the last six months of the year and the spouse pays more than half the cost of keeping up a home which is the main home of his or her child).Now, if the spouses became married on or after January 1 of the current year, they still file as single for the previous year since they were considered unmarried as of December 31 of that year, even though they may be considered married when the returns are actually filed the following year. But otherwise, they cannot use the single filing status.