Do I answer yes to the question "is the amount in Box 2A my taxable amount" or do I answer No because I have employee contributions in box 9B?

[DanielV01]

It depends.  If Box 2a = Box 1 minus Box 9B, then you will answer yes because your previously taxed distributions have already been accounted for.  However, if box 2a is either blank or matches box 1, you report no.  The amount of employee after-tax contributions must be reconciled in box 9B to determine the taxable portion of the pension or annuity that you are receiving.

[hammerinone]

If taxable income did not reflect the 9b deduction, and I forgot to do it last year, my first year of retirement, can I do it this year even though I am not prompted?

[dmertz]

Box 9b cannot normally be used in calculating the taxable amount because it indicates that total amount of after-tax contributions to the plan, not just the portion of after-tax contributions included in the distribution.

If box 2a has an amount in box 2a and box 2b Taxable amount not determined is not marked, the amount inbox 2a is most likely the correct taxable amount and the IRS will question any amount that you indicate as the taxable amount that is lower than the amount in box 2a.  The amount in box 2a is generally the amount in box 1 minus any amounts in boxes 5 and 6.