Many foreigners underestimate the particles in Korean or Japanese. They aren't just a single syllable that tells you about the case, but more than often, the nuance of the whole pharagraph depends on a single particle. You should pay good attention, especially if you see 은/는 like in your example. 은/는 is indicative for counterpart/antithesis Below is how I would translate the example to English: 민수: I play basketball or soccer. I especially like basketball a lot. 투이: Wow, you must be really good at it then 민수: Well, not exactly. Now guess, where does this "not exactly" come from? It's from the particle "는": - You remind that there is a counterpart/antithesis to 잘하다(to be good at), it being 좋아하다(to like) - You also remind that these are two different things/stories ==> "Well, that's another story" "잘 하지 못해요" or "잘 못 해요" would simply mean "I'm not good at it", but the partilce "는" adds so much depth to the whole paragraph implying: - My ability might be above a certain level, but you would be disappointed if you expected too much. - I can't say that I suck, but I'm not particularly good either - Oh please...... I might have been playing a lot, but I'm not talented at all.

if you simply say 잘 못해요, it means just as it is - you are not good at it. But in this context, the speaker wants to say that he likes Basketball BUT he is not really good at it. "는" of 잘하지"는" 못해요 adds this nuance that one likes something but does not necessarily do it well. For your information, this kind of conversation could be quite common especially someone is trying to be modest about their talent. Sometimes you will find someone says that he or she is not good at something but actually very good at it.

...을 잘하지(는) 못하다 means " don't (doesn't) do well ". ...을 잘하지 못하다 = ...을 잘 못하다 = 훌륭하지 않다 잘하다 : 실력이 훌륭하다 ex) 수학을 잘한다 = 수학 실력이 훌륭하다 Negative pattern of " 잘하다 " is " 잘 못하다" 잘하지 못하다 : 훌륭하지 못하다 ex) 수학을 잘 못하다 = 수학을 잘하지 못하다 = 수학 실력이 훌륭하지 않다