When you are actually engaged in the action.
I am studying Korean (at this exact moment I am engaged in studying Korean)
저는 한국어를 공부하고 있어요
I am eating rice
저는 밥을 먹고 있어요
I mostly remember it myself by thinking I could use '지금(은)' in the sentence.
저는 (지금은) 한국어를 공부하고 있어요
저는 (지금은) 밥을 먹고 있어요
But, if you want to say: I'm studying Korean (general remark, but not a current action)
저는 한국어를 공부해요