whenyoudaoicibaDictYouDict[when 词源字典]
when: [OE] When was formed from the ancient interrogative base *qwo- (source also of English what, who, etc) and a nasal suffix which also appears in then. From the same ancestor, but with an additional dental suffix, came Latin quando ‘when’, source of English quandary.
=> quandary[when etymology, when origin, 英语词源]
when (adv.)youdaoicibaDictYouDict
Old English hwænne, hwenne, hwonne, from Proto-Germanic *hwan- (cognates: Old Saxon hwan, Old Frisian hwenne, Middle Dutch wan, Old High German hwanne, German wann "when," wenn "if, whenever"), from pronominal stem *hwa-, from PIE interrogative base *kwo- (see who). Equivalent to Latin quom, cum. As a conjunction in late Old English. Say when "tell me when to stop pouring you this drink" is from 1889.