You use Form 843 only if the employer refuses to refund the improperly-withheld Social Security and Medicare tax.
In the process, the FIRST step is to notifiy the employer of the error in withholding these taxes and formally request that they be refunded. This can be done with minimal effort on the employer's part up to the end of January 2007.
If the employer refuses, for whatever reason, you need to get the refusal in writing. Once you have that, you can then download IRS Pub 519 to read pages 42-48 and Form 843 to request the refund. See
Internal Revenue Service.
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