Corporations file Schedule M-3 (Form 1120) to answer questions about their financial statements and reconcile financial statement net income (loss) This form is for general instructions that apply to all corporation tax forms.It includes a variety of topics about how to fill out your form. Each state may use different regulations, you should check with your state. Where to file your taxes for Form 1120-S ; A foreign country or U.S. possession. Form 1120 is the corporate income tax return form that US corporations need to file with the Internal Revenue Service (IRS). You have to first form a C corp. Once the business is a corporation (C corp), all the owners must sign and file Form 2553 to elect S Corp status. Form 1120-S is considered the tax return of an S corporation.