a. 졸려서 일을 잘 못해요 Translation: I am sleepy so I cannot do my work well. note: 일 has a wide range of meanings and usually connotes something that includes work, like homework/housework/office work b. 일을 잘 못하겠어요 Translation: I can't do my work well [in this moment] note: The grammar form 겠다 adds on a meaning of urgency or in this particular situation. In the first example sentence, 졸려서 일을 잘 못해요, you are stating the fact that you are sleepy so you can't work well. But, if you were to say 졸려서 일을 잘 못하겠어요, you would be saying that in that moment, there's no way that you could work well. It adds onto the immediacy and degree of the ability/inability to do something. c.잠을 잘 못 잤어요 (almost cannot sleep or cannot sleep well? Is it same with 잠이 못 와요?) Translation: I couldn't sleep well note: if you were to say 잠을 못 잤어요, that means you couldn't sleep at all, but adding the 잘 before the 못 implies that you could sleep, but not well. 잠이 못 와요 is very similar to 잠을 못 자요, but is the passive version of the sentence. Thus, it sounds more like you're lying in bed and the sleep won't come to you versus you actively can't do the action of sleeping (it's a very fine difference, one that you shouldn't have to worry about in conversation) 2) "지만"에 대해서 건데 언제 이/가 쓰는지 은/는 쓰느지 잘 몰라요 ㅜㅜ 이/가 and 은/는 are really best learned by being in conversation and listening to how native speakers use it. In general, 은/는 is used when comparing something to something else, and denotes a topic. 이/가 are just regular subject particles to denote what is the subject in the sentence versus the object. I suggest you check out Talk to me in Korean's lessons and other resources to read more about it! :) d. 그 아이는 똑똑하지만 성격이 나빠요. in this case, you sound like you're comparing this 아이 with another 아이 that you have in mind but if you wrote the sentence like this: 그 아이가 똑똑하지만 성격이 나빠요. you don't really have the comparison aspect to the sentence and sound like you're just talking about this child being smart but bad