“I have just learned that $16,000 paid to me from a trust fund in early 2010 should have been declared on my 2009 return... oops! How do I go about correcting this?”---->In general, The IRS usually corrects general math errors and simply lets you know of any changes. But you need to file your amended return as you discover ,before the IRS does, that your total income was reported incorrectly(understated by $160,000 in this case): you need to use form 1040X to correct previously filed 1040 or your state return form. Your amended return is done on paper.So, your Form 1040X must be filed by mail; the IRS does not allow it to be efiled, so you must print your tax return and mail it with your completed Form 1040X to the IRS address provided in the 1040X instructions. As long as you can pay the extra tax by April 18th 2011, there will be no penalties or interest due. Any debt remaining past April 18th 2011 will be subject to penalties and interest for late payment of tax. You MAY ask the IRS to waive the penalties and sometimes they'll do that if you have a good reason; I guess you may contact IRS nearest you in you r local area.