Except in very specific circumstances, married people cannot file their income taxes with a status of "single." If you're married, you usually have only two options at tax time: file a joint tax return, or file as married but with separate returns. The latter option in some ways resembles filing as single, but it also eliminates some big tax breaks.
When You're Married
If you were married as of December 31 of a year, then the IRS considers you to have been married for that entire year. Even if you got married on December 30, and even if you earned every cent of your income that year while you were still single, you are married as far as the tax code is concerned. When you prepare your taxes for that year, you must use one of the married filing statuses.
When You're Single
By the same token, if you were unmarried on December 31, then the IRS views you as having been unmarried for the entire year. Say you got married and neither of your were married before on January 1. When you prepare your taxes for the previous year, you must each file your own returns as single, because that was your status on December 31 -- even though you're married the next day.
There is only one circumstance under which a couple who were legally married on December 31 can file as single for that year. If, at year's end, you were legally separated from your spouse under a divorce decree or a decree of separate maintenance, then the IRS says you may file as single. The definition of "legal separation" is a matter of state law, however, and the IRS will consider you legally separated only if your state does. Be aware that simply living apart isn't enough. You need a decree -- an order, ruling or judgment from a court -- recognizing the separation. If you are divorcing, but your divorce was not yet final on December 31 and you were not legally separated, you were not single, and you must file using a married status.
Filing separate returns allows a married couple to treat their incomes independently for tax purposes -- kind of like being single. In some circumstances, this can lower your tax bill. For example, unreimbursed job expenses are tax-deductible only when they exceed 2 percent of adjusted gross income, and medical expenses are deductible only when they top 7.5 percent of adjusted gross income. If one spouse has a lot of these expenses but relatively little income, then that spouse might be able to claim a big deduction on a separate return, but only a small deduction -- or no deduction at all -- on a joint return. However, married couples who file separately are not allowed to claim a lot of popular tax breaks, such as the deductions for IRA contributions and student-loan interest, the earned income credit, and the tax credit for childcare expenses.