If you know that you need to make changes to an income tax return that has already been filed, you should remember that not every mistake will require you to file an amended return. For example, if you realize that you made a math error on your tax return, you can presume that the IRS will make the correction for you. If you neglected to attach standard forms to your tax return, like a W-2 Form or certain schedules, you can wait for the IRS to request them from you.

In general, the IRS expects all the information you provide on your (initial) tax return to be accurate and complete when you file it. Therefore, the IRS is unlikely to hold you responsible for information that did not come to light until after you filed your return. Nonetheless, you should still consider submitting an amended return, especially if doing so may reduce your tax liability and/or increase your tax refund.

You may also want to file Tax Form 1040X if you are concerned about being penalized for failing to report income, or for claiming tax deductions or exemptions to which you later discover you were not entitled. By correcting an error or omission before it is found by the IRS, you can help reduce any penalties or interest that you may owe for underpayment of taxes. For instance, you may want to use a different filing status, change your personal exemptions, add or subtract dependents, report additional income or withholding tax or adjust your tax deductions and/or tax credits.

If you are filing a 1040X Tax Form, you are not required prepare a revised 1040 Tax Form. Even so, you may still want to start from scratch using a fresh 1040 Form when making any changes to your tax return. This is because you might find, for example, that an adjustment in your income also alters your eligibility for certain tax deductions and tax credits, based on income thresholds. 

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If you are filing a 1040X Form to claim an additional tax refund, you must wait until you have received your original tax refund before sending in an amended return. You are allowed to cash the first refund check while you are waiting for any additional tax refund. If you owe additional taxes or you need to return a portion of a tax refund, though, be sure to file Form 1040X and pay the tax as quickly as possible in order to reduce the interest and penalty charges that can accrue.

When amending your federal income tax return, keep in mind that your state income tax liability may also be affected. Contact your state taxing authority for information regarding whether you need to amend your state tax return as well as how to amend it.