It depends of what you wanted to say : " I must study more korean ' or" I must study korean more " but i think it is : 한국어 공부 더 해야 지요 = I must study korean more - hard - and in that case you add the adverb '열심히' => 한국어 공부 더 열심히 해야 지요

Hi Ake Hae, '더 한국어를 공부해야지요.' is supposed to be '한국어를 더 공부해야지요.' to be in right order. And '한국어 공부를 더 해야죠' may sound more natural. It means 'I need to study Korean more.' (Because the subject is not specified, I suppose it is 'I'.) Also, you can also say '한국어 공부를 더 열심히 해야죠' for 'I need to study Korean harder.' Hope it helps.