Again, the writers of "Du's Handbook" do not explain how they translated the above Chinese sentence, but they seem to have translated it as an "if...then" sentence, with 而 (이) representing the "then" portion. Also, they seem to have translated 能 (능) as a full verb meaning "can do," instead of just an auxiliary verb meaning "can." Finally, they seem to have translated 之 (지) as the pronoun "it." The problem I have with the translation is that I have never seen 能 translated as a full verb meaning "can do." Therefore, I want to suggest the following translation:不賢而能之與?"Could he have done it if he had not been a worthy man?"
As you may be able to see, I have translated both 賢 (현) and 能 (능) as adjectives jointed together by the conjunction 而 (이), meaning "and." The 不 (불) negates the question, not either of the adjectives, creating a kind of negative tag question, which could be translated in English as follows: "He has been wise and capable, hasn't he?"不賢而能之與?"Has [he] not (不) been wise (賢) and (而) capable (能之與)?"
The 與 (여) is essentially a question mark, supposedly the same as the question mark 乎 (호), but I am guessing that the combination of 之與 (지여), working together with 不 (불), forms a negative "tag question," which is the "hasn't he" in sentence above.
In Korean, I think it should be translated similar to the following: "그는 현명하고 유능하지요? 그렇지 않아요?
Again, I could be way off base since I have not seen this sentence translated the way I have just translated it, but the translation in "Du's Handbook" bugged me so much that I just wanted to suggest something new. If anyone has a different translation, please let me know.