I'm surprised that the accountant didn't inform you that you would need to amend 2018/2019 if you had a 2017 capital loss carryover. the 1256 losses, reported on form 6781, carry to schedule D. only then can you determine if you have a capital loss carryover. you would if there's a loss on line 16 of schedule D that exceeds the loss reported on line 21
if you have a CLCO from 2017 you must determine the components - short-term CLCO and/or long-term CLCO by following these rules.
You're allowed to use an unlimited amount of capital losses to offset any capital gains that you incur in the same year. First, you'll separate your gains and losses by holding period, with those on investments you owned for longer than a year in the long-term category and those on investments you owned for a year or less in the short-term category.
If you have a gain in one category and a loss in the other, you can then use the losses to offset the gains. Any net amount remaining stays in its existing category. If you have losses in both categories, then you'll keep them in their respective places in applying the next step.
The $3,000 rule
Once you've offset all your capital gains, you can use an additional $3,000 of capital losses to offset other types of income, such as wages and salaries or investment income. If your losses amount to less than $3,000, then you simply take your remaining losses and have nothing left to carry over.
If your losses exceed $3,000, then you have to look further. If you have short-term capital losses of $3,000 or more, then you'll take all $3,000 from the short-term category. Your carryover amount will therefore be any remaining short-term losses along with all your long-term losses. If you have less in short-term losses, then first use any short-term losses you do have, and then take long-term losses to take you up to the $3,000 total. Any remaining long-term losses in that situation are eligible to carry over.
once your capital loss carryover, if any from 2017, is calculated, you must carry it to 2018, you can't skip years and carry a 2017 capital loss carryover directly to 2020. in 2018 you calculate like above. any remaining capital loss is carried to 2019. any remaining from 2019 is carried to 2020. so if you have CLCO from those years you must amend 2018/2019
there will only one form 6781 for 2020 which will only show 2020 activity. as stated the 1256 losses become part of capital losses. they are only reflected on schedule D not form 6781
