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Make a mistake on your tax return? That's OK -- file an amended return.

What is an amended tax return? It's your do-over, your mulligan, your second chance. The IRS recognizes that we sometimes make mistakes on our tax returns, or that our circumstances can change, so we're allowed to file amended tax returns -- via IRS Form 1040X (PDF file).

When to file an amended tax return
Believe it or not, discovering a math error on a return you filed doesn't mean you need to file an amended return. The IRS routinely spots and corrects errors and will either adjust your refund, or send you a bill for any extra funds due. If you forgot to include a particular form or schedule, you still may not need to amend your return. The IRS will likely notice it's missing and send you a letter asking for it. (Beware of scams, though: The IRS doesn't phone or email taxpayers, so don't respond to any such communications.)

Examples of when you do need to file an amended return include:

  • Your filing status or income for the tax year changed. (Perhaps you forgot to include some income you earned or received a 1099 form detailing some dividend income after you filed your original return.)
  • The deductions and/or credits you're claiming for the tax year have changed. (You might not have realized that you could deduct a certain expenses or claim a particular credit, or perhaps you claimed one you weren't entitled to claim.)
  • You realize you owe more tax than you paid. (If you realize this before the tax filing due date, usually April 15, has passed, you can file an amended return with the extra payment and avoid getting hit with a penalty.)
  • You failed to claim a dependent, or claimed one you shouldn't have.
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Form 1040X lets you correct mistakes. Source: Marcin Wichary, Flickr.

How to file an amended tax return
You'll need to file a separate Form 1040X for every tax year's return you want to amend -- and each one must be mailed in separately. In most cases, you must file your amended return within three years after the date you filed your original return, or within two years after the date you paid the tax, whichever is later. If you amend your return for the last tax year before the deadline for that year, it's considered filed on time.

Form 1040X cannot be filed electronically. You must submit it on paper. Tax-prep software can help you fill out the paperwork, if you use such software.

The main portion of Form 1040X consists of three columns, labeled A, B, and C. That's where you'll note most changes. Column A is for numbers filed on your original or last-amended return, column B is where you note additions or subtractions, and column C is where you report the correct amount for that item. Along with the form, submit any additional forms or schedules relevant to changes you're making. For example, if you're changing itemized deductions, submit a new Schedule A. If you're changing your reported income, include a copy of the relevant W-2 form.

On the back of form 1040X, you'll need to briefly explain why you're filing an amended return.

Note that if you're someone who has to file Form 8938 (Statement of Specified Foreign Financial Assets), then you must file it with your amended return, too. Note, too, that if something on your federal tax return needed amending, there's a chance you'll need to amend your state return, too. If so, your state will probably require a copy of your form 1040X.

Once you file your amended federal tax return, if you're anticipating a tax refund, know that it typically takes up to three weeks for amended returns to show up in the IRS's system, and up to 16 weeks to be processed. You can check the status of your amendment via the IRS's Where's My Amended Return? Service.

And finally, a few cautions: If you're amending your return, you must include all changes, whether they increase or decrease your tax bill. You can't just change numbers that will benefit you. And... amended returns are more likely to be audited.

Longtime Fool specialist Selena Maranjian, whom you can follow on Twitter, has no position in any stocks mentioned. The Motley Fool has no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.