던 and 았던 -던 - describes an action or event that was happening in the past and which might still be going on or resume. -았/었던 - describes an action or event that went on in the past which completed and can't resume anymore. (the action -던 represents may or may not have ended, while -았/었던 is clear about its status of having ended) Examples) - 그거 내가 먹던 케익이야 - That's a cake I was eating. (the speaker might still want to eat more). - 그거 내가 먹었던 케익이야 - That's a cake I already ate some. (he touched it, but no longer wants it). - 내가 옛날에 살던 집 - The house I lived in long ago (a case where the action ended) - 내가 옛날에 살았던 집 - same. In these examples, there is little difference in meaning (because of 옛날에). 다들 잘하려니 하고 내버려 두었는데 그다지 결과가 좋지 않았다. => I left them alone trusting they'd all do well, but the result turned out not that good. 1.잘하느니. -느니 = (rather) than (집에 있느니 공원에라도 가자 - Might as well go to a park rather than stay home) 2.잘하겠거니. -겠거니 = thinking/assuming something would turn out ... (곧 낫겠거니 생각해 = I think I'd get better soon) 3.잘하니만큼. -니만큼 = given that ...; as/since. (프로 선수니만큼 잘한다 = He's indeed good as he's a professional athlete) 4.잘한다든가. -다든가 = does/is something for instance, and/or .... (수업 중에 존다든가 떠드는 사람은 혼난다 = Anyone who doze off or chat during class will be punished) So the answer is (2), as -려니 and -겠거니 have the same meaning of making a guess or conjecture.

* 정답은 2번 '잘하겠거니'입니다. '기대'나 '추측'할 때 쓰는 용법입니다. '잘하겠거니' 대신에 '잘하겠지'를 써도 됩니다. 각각의 의미와 용법: 1. 잘하느니 : ' -느니' * 동사 어간+-느니 : 잘하+느니->잘하느니, 먹(먹다)+-느니->먹느니 * 비교하여 선택할 때 씁니다. (차라리) rather, as soon, sooner than 항복하'느니' 차라리 죽는 편이 낫다. I would sooner[rather] die than give in. 3. 잘하니만큼 : '-니만큼'('잘하니까'로 바꾸어도 됩니다.) * '원인'이나 '근거' 학교가 머니만큼 일찍 나가야 한다. ( 머(멀다: 멀고, 머니)+니만큼) 4. 잘한다든가 : '-든가'('든지'로 바꾸어도 됩니다.) * '선택'을 할 때에 씁니다. either...or, whether...or 밥이 없으니 점심으로 떡을 먹'든가' 빵을 먹'든가' 해라. 네가 가'든지' 말'든지' 내가 알 바 아니다. Whether you go or not, I don´t care about it. 그가 죽'든지' 살'든지' 난 관심 없어. I don´t care whether he lives or die * 영어 예문은 네이버 영어사전을 참조했습니다. http://endic.naver.com/