You can carry back 1 year (2016) if it's feasible. For tax year 2016, if you used the exemption and did not fill out a form 1116, you cannot carry back to 2016. If you filled out a form 1116 for 2016 and the foreign tax credit limit is greater than the foreign tax used; (line 21 is greater than line 14) you can carry back up to the limit. To do this, you would file a 1040X and a revised form 1116 for 2016.
