There are several reasons one may need to file an amended tax return. If you have worked more than one job, or if you have worked as a sub-contractor or independent contractor, there may be a W-2 or a 1099 that you didn't receive until after you had filed your taxes.
E-filed returns cannot be amended online. You must file a paper form 1040X for the year you are changing. You can go back no more than three years to make changes. Residents of certain disaster areas and those who can prove to be unable to take care of their own finances may be able to make changes outside of the three year time space.
The 1040X form is simple to complete. There are basically three columns of figures to be completed. The first column asks for the original amount that you put on the original 1040, 1040A or 1040EZ. The center column (column B), requests the amount of the increase or decrease. Column C asks for the current or new amount that the return should have had on it.
For example, if your original return showed $22,000 from all W-2's and the forgotten W-2 is for $2500, then on Line 1 you would put $22,000 in Column A, $2500 in Column B and $24,500 in Column C. Adding together each line straight across, Column A plus Column B should equal Column C. If they don't match, then there is a mistake somewhere in the figures.
All the other lines should be completed the same way.
If you are changing the current year, you can wait until the return has been processed and then file the 1040X or you can send the 1040X as soon as you realize the mistake.
Additionally, the 1040X form will ask for an explanation of the changes. You then simply list the line number that was changed and explain why. Usually it's as simple as writing: Line1- Forgot W-2 (or forgot 1099), and/or Line 11- Tax withheld on new W-2.
If you took a deduction for the Additional Child Tax Credit, you will have to fill out a new form 8812, using the new amounts and include it with the 1040X.
The final figures at the bottom of the 1040X form will show how much more you owe or how much more you can expect to receive as a refund. If they show that you must pay additional tax, you can either include the payment with the 1040X or wait for the IRS to send you the bill notice. If you are owed an additional refund, the IRS will notify you by postal mail of when you should receive it.
The 1040X and the 8812 can be printed from the IRS website.