S corporations: File a 2011 calendar year income tax return (Form 1120S) and pay any tax due. Provide each shareholder with a copy of Schedule K-1. If you want an automatic 6-month extension of time to file the return, file Form 7004 and deposit what you estimate you owe.

S corporation election: File Form 2553, Election by a Small business Corporation, to choose to be treated as an S corporation, beginning with calendar year 2011. If Form 2553 is filed late, S treatment may not begin until calendar year 2012

Make your first installment of 2011 income tax on Form 1040-ES to pay income tax you expect to owe in excess of withholding.

Partnerships: File a 2011 calendar year return (Form 1065). Provide each partner with a copy of Schedule K-1 (Form 1065). If you want an automatic 3-month extension of time to file the return and provide Schedule K-1, file Form 8736.

Corporations: Deposit the first installment of estimated income tax for 2011.

Corporations: Deposit the second installment of estimated income tax for 2011.