오늘은 한국어를 많이 공부했어요. -> It is Ok. But I think..오늘은 한국어 공부를 많이 했어요. -> It is better.^^
Diffence between the two sentences is the first one means among the a lot of subjects you spent most time to "한국어". And the second one means you spent much time to study Korean not to compare with anything.
오늘은 한국어 공부를 열심히 했어요.
꼭 고치자면, 이렇게 말하는게 더 적절해요.
오늘은 한국어를 많이 공부했어요. -> It is Ok. But I think..오늘은 한국어 공부를 많이 했어요. -> It is better.^^
Diffence between the two sentences is the first one means among the a lot of subjects you spent most time to "한국어". And the second one means you spent much time to study Korean not to compare with anything.