Who must file Form IL-1120? You must file Form IL-1120 if you are a corporation that.Corporations file Schedule M-3 (Form 1120) to answer questions about their financial statements and reconcile financial statement net income (loss) This document is sometimes referred to as the charter. In this guide, you'll find information on naming your corporation, getting a Registered Agent, the fees you'll need to pay, business taxes and much more.