TurboTaxLina
, MST, EA hasn't provided a complete answer here.
You receive a Form 3921 when you exercise an ISO. The Form 3921 helps determine the basis of the stock when you actually sell the stock.
You don't "enter it on your income tax return" in the sense that the IRS will receive the Form 3921 as part of the income tax return itself, but you might want to enter it in TurboTax when you do sell the stock if you use the ISO "Guide me" interview because that's how TurboTax helps you determine the correct basis to use.
But, more importantly, if you exercise an ISO and don't sell all the stock before the end of the year of exercise then you MIGHT be subject to the Alternative Minimum Tax because the "spread" between what you paid for the stock and the value of the stock at exercise IS an AMT adjustment item. In this case the Form 3921 should be entered into TurboTax using the "ISO Exercise and Hold" interview. That's the interview that makes the entry needed on Form 6251 where the AMT is calculated. Even in this case you're not literally "entering it on your income tax return" but you DO need to enter it in TurboTax using this interview to see if you're subject to AMT. That's a point that often gets overlooked if you don't know to do it, and I'd guess you didn't know to do it or you wouldn't have asked the question.
As to your question "What do i do?" I'd say you open your income tax return and enter the Form 3921 in the "ISO Exercise and Hold" interview. If the "refund - o - meter" at the top of the screen doesn't change then you close your income tax return without saving it and rejoice. If the "refund - o - meter" indicates an increase in your tax liability then you probably have fallen into the AMT, and need to amend.
Tom Young