Form 1120S (2018) is used by S corporations to report their income, gains, losses, deductions, credits, and other information. The form should not be filed unless the corporation has filed or is attaching Form 2553 to elect to be an S corporation. The form has several sections, including Tax and Payments, Deductions, and Other Information. The form also includes schedules for reporting additional information. The form must be filed by the 15th day of the third month after the end of the corporation's tax year.