Global Income Tax Return in Korea(종합소득세)
Residents who earned income in Korea need to file the captioned return by the end of May, you can visit a tax office near you with your ID and a bank account info. if you had any chance to get refund.
There was around 143bil.won refund amount is parked in BoK as of last May, so you can try to get back some.
You can apply back tax 5 years only so why don't you try it back in this May.
You can download those earned income from the Home Tax(www.hometax.go.kr) with some help with your Korean friend, and file it through the mobile phone.
If you paid a 3.3% tax to your employer, You must file it without any exception even you already did it with your employer for "Year-end settlement(연말정산)" of salary income, since 3.3% tax was for a Business income, so you need to file while combining 2 income sources(Salary income and Business income) even with the same employer.
You can also file again if you doubt on your "Year-end settlement(연말정산) which was done by your employer in February.
If you didn't file it on time, the penalty is huge, you need to pay 20% more than you need to pay, and a late payment penalty will be added.
Please send me an email if any questions to email@example.com