Every corporation(around 790,000 company) must file the Corp. Income Tax Return and pay the tax by the 1st of April for 2018 FY.
As I stressed 100,000 times, please keep the due date for filing in any circumstances.
You can file by your self though the "Home Tax"-www.hometax.go.kr
Almost all of SME are entitled to deduct a kind of special exemptions from 10-30% of calculated tax up to 100mil.won if it's not a specific business scope like a restaurant or private institute.
If you didn't file on time, you can't apply the above mentioned deductions in anyway, so do not miss the date please.
Any questions, please feel free to send me an email to email@example.com