I had a capital loss in 2009 that wasn’t reported. Can I claim it now?

A capital loss must be applied in a set order at a set time. In the year in which it is realized, it offsets capital gains and, if there is any excess loss, up to $3,000 of ordinary income (such as salary and bank interest). Unused losses are then carried over and applied in the same way for that year. And so on and so on. You can’t opt to use a 2009 loss in 2013 without following the rules for all intervening years.