Where the Internal Revenue Service is concerned, you're either single or married – you can't be both. If you're married, you can't file as a single taxpayer, and if you're single, you can't file as a married taxpayer. If you made a mistake, however, you can go back and fix it. It's far better to voluntarily amend your return than to suffer an audit if filing wrongly resulted in a tax benefit to you that you didn't deserve.
The last day of the tax year – December 31 – determines if you were single or married for the entire year. If you got married on New Year's Eve and said "I do" before midnight, you can't file a single return. If you filed as single because you didn't understand the tax rules and figured you weren't married the majority of the year, you can amend it to either a separate married return or to a joint married return if your spouse agrees to file with you.
You might also want to amend your return if you filed as a single taxpayer because you thought you weren't married anymore and you now realize that you were – at least in the eyes of the IRS. Informally separating from your spouse doesn't mean you're single, even if the two of you have signed a separation agreement. You can only file as single if you have a divorce decree or an order from the court that legally separates you before the last day of the tax year. If you thought otherwise and filed as a single person, you should amend your return.
Head of Household
The IRS offers another option if you and your spouse were living apart at the end of the tax year but weren't formally divorced or separated. You're considered unmarried, even if you were still legally married, if you meet certain criteria. If you and your spouse lived separately from July 1 on, if you paid for more than half the expenses of your home and if you have a dependent, you might be able to file as head of household. This usually results in a more tax advantages. You can amend your single return to a head of household return just as easily as you can amend to a married separate return, but check with a tax professional first to make sure you qualify because the rules for qualifying are specific and complicated.
Amending Your Return
Amending your return is relatively simple, but you can't just submit a new Form 1040, 1040EZ or 1040A. You must submit Form 1040X, which is an amended return. You can change your filing status on this form, report your same income, then take any tax credits or deductions you qualify for under your new filing status. You have three years to amend your return, beginning from the tax due date. If you filed your single return in February, you have three years from April 15 of that year to change your status.