When to File

• If you discovered, after you filed your return, that you forgot to attach a schedule or you made a mathematical error, you may not need to file an amended return. The IRS corrects mathematical errors and recommends that you only file an amended return if your schedule or form omission changes your income, filing status, deductions or credits, for example if you received another W-2 Form after you filed. To amend your return, you file a 1040X Form.
  
  

Rules

• The filing rules differ depending on whether you are expecting an additional refund or you owe more. If you are filing because you forgot a credit or deduction that will net you a refund or an additional refund, you cannot file your 1040X until you receive the original refund, if there was one. You may cash the original refund check.
  
  

Filing the Form

• You must file a separate form for each tax year you are amending, and each form must be mailed in separate envelopes. If you expect a refund, you have three years from the date you originally filed your original return or two years from the date you made your last payment, whichever is later. If you filed your return before the April due date, it is considered filed as of the April due date.
  
  If you expect to owe more money, you must file the 1040X, along with any additional forms or schedules that are needed and pay the tax by April 15 of the following year to avoid any interest or penalties. If April 15 falls on a Saturday, Sunday or legal holiday, the date is extended to the following business day.
  
  

Information Needed

• To complete the 1040X Form, you will need information from your original tax return. The 1040X has three columns -- one for information from the original return; one for the new information; and one for the difference between the two. On the form's reverse, you must describe the change and the reason for the change, for example -- my earnings increased because I filed before I received the attached W-2. You must attach the forms and schedules that the change affects, including any new W-2 Forms. You cannot e-file an amended return.