Single is the default filing status for taxpayers who are unmarried or considered unmarried.
Your marital status on the last day of the calendar year determines your filing status for the entire tax year. You can file as single if you were unmarried or legally separated from your spouse under a divorce or separate maintenance decree on the last day of the year. Likewise, if you got married on or before December 31, you cannot file as single for the tax year.
Unmarried taxpayers who support a dependent and maintain a home should check to see if they qualify for head of household status or qualifying widow(er) with dependent child status, both of which features several tax benefits over filing as single.
Also, the federal government does not consider domestic partners or same-sex spouses legally married and thus they must file their tax returns as single.