Knowing the context of the above sentence might satisfy my curiosity, but since I do not know it, I want to speculate. "Du's Handbook" explains (on Page 55) that "when an 而 (이) comes between a subject and verb, it may imply a conditional 'if.'" The fact that "may imply" was used in the explanation suggests to me that the writers are either unsure or that there are times when it is not the case. However, let's assume the writers of "Du's Handbook" translated the above sentence correctly.管氏而知禮, 孰不知禮?“If [even] Mr. Guan knew the rules of propriety, who would not know them?"
For someone who does not speak Korean, putting the "if even" between the subject and the verb might seem strange, but for someone who speaks Korean, it is not strange at all. In Korean, the 而 (이) would translate as "까지...면," so in Korean the sentence would translate as follows:
관씨까지 (管氏而) 예절을 알면 (禮知), 누가 (孰) 예절을 못 알겠는가 (不知禮)?Perfect! Right? Nevertheless, I want to suggest one less likely possibility:
In a Korean dictionary, one of the meanings of the character 而 (이) is 능 (能), which means "can," but 能 can also mean "might" or "possibly." After all, the Korean word for "possibility" is 가능 (可能). Therefore, in cases when the 而 (이) comes between the subject and the verb, I am wondering if there might be times when either the auxiliary verb "might" or the adverb "possibly" are better translations.管氏而知禮, 孰不知禮?Mr. Guan (管氏) might (而) know (知) etiquette (禮). Who (孰) does not (不) know (知) etiquette (禮)?