Back Tax in Korea
With a various reason, we need to file for past years, in Korea you can file for the last 5 years at this moment(2016, 2015, 2014, 2013, 2012 FY), even for 3 years, you can do it through the "Home tax" system by yourself.
You can check all of your income, withheld amount, paid amount and arrears if you had through the Home Tax System once you break through the notorious "Active X".
If you paid more than you need to pay before, you may apply some refund(경정청구) and you will get it in a 3 months but it depends on your district tax office situation.
Most of cases for claimed tax bills issued by the tax office, it's hard to apply for refund or a late filing practically, I mean you need to file for back tax before you got the tax bill from the tax office.