You must file Form IL-1040-X, Amended Individual. Income Tax Return, if.Forms on this website are PDFs with fillable fields. Forms should be downloaded to your computer before filling in the fields. Illinois Legal Aid Online provides a guided interview called an Easy Form that will help you fill out the forms. Not all forms are listed. Gov for questions regarding completion of this form. All forms are ADA accessible, and you can handwrite or type into the PDF form to fill it out. Upon receipt of the summons, complete the information form and return it to the U. S. District Court within five days in the postage paid envelope provided. Illinois law provides the Certificate of Error procedure to allow the Cook County Assessor to apply changes to a property tax bill that has already been issued.