Tax forms can be long and complicated.

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You sign your Form 1040, seal it up in an envelope and send it off to the Internal Revenue Service two weeks before April 15. You watch all your fellow taxpayers panic as April 15 arrives. Then everything is quiet on April 16. Another tax season has come and gone. The only reminder will be that nice refund check that will arrive in your mailbox one sunny afternoon.

But then, on April 29, you're shuffling through your file cabinet when you come across a W2 you don't recognize. It's a statement of wages from the old job you still had at the beginning of the year. You'd forgotten about that job and the W2 from the employer. You realize you didn't report this income on your Form 1040.

Pop quiz, hot shot: What do you do? It's time for the 1040X: An amended U.S. individual income tax return.

­When you need to make a correction to your Form 1040, as in the above example, you can fill out a Form 1040X to show the changes and explain the reasons for those changes.

In this article, you'll read about some of the changes you can make to a return. Then you will learn how to fill out Form 1040X to make these changes. Finally, you'll learn about the time restrictions the IRS places on amended tax returns.