When you are actually engaged in the action. E.g. 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