Do not file another Form IL-1040 to make changes to a previously filed Form IL-1040. This affidavit is for victims of identity theft.To avoid delays do not use this form if you have already filed a Form 14039 for this incident. You must file Form IL-1040-X, along with proper supporting documentation, for both corrected and amended returns. Filing: If you are efiling any of these forms, you will need to "flatten" the form so it cannot be changed after you complete it. Forms on this website are PDFs with fillable fields. Forms should be downloaded to your computer before filling in the fields. After the form is submitted to the Office of the Illinois Treasurer, any changes to information on the form must be communicated in writing. Both employees and employers (or authorized representatives of the employer) must complete the form. Use our Form Filler to fill out your form and print it.