When to File as Single
Filing status is determined on the last day of the year. Anyone who is not married as of December 31 must file as single. This means that if you were married for part of the year, but are legally divorced or separated before December 31, you are required to file as a single taxpayer. This is true regardless of how late into the year the divorce or separation becomes final. The sole exception to this rule is if you divorced for the sole purpose of filing as single with the intention to remarry the next year. Under this circumstance, you must still file as married.
When to File as Married
If you are married as of December 31, you must file as married. If you have filed for divorce or legal separation but the divorce or legal separation is not final by December 31, you must still file as married.
Whether to File as Married, Filing Jointly or Married, Filing Separately
Assuming you are entitled to file as married, you may choose to file jointly with your spouse or separately. While it is generally advantageous to file jointly, this is not true all the time. You should compute your tax liability under both separate and joint filing to see which one is best in your situation.
What happens if your marriage was annulled?
If your marriage was annulled prior to December 31, you must file as single. Since an annulled marriage legally never existed, you are also required to file amended tax returns for the three previous years, On these amended tax returns, you should file as single.