The grief of a fallen kingdom defies words — only a line like 'when will spring flowers and autumn moons end?' can express it.
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history
Opening line of 李煜《虞美人·春花秋月何时了》(Li Yu, last ruler of Southern Tang, written c. 978 while held captive in the Song capital): 春花秋月何时了？往事知多少。小楼昨夜又东风，故国不堪回首月明中……问君能有几多愁？恰似一江春水向东流 (When will spring flowers and autumn moons come to an end? How much of the past can one know? Last night in the little tower east wind blew again — I cannot bear to look back at the lost kingdom under the moon... How much sorrow can I bear? Just like a river of spring water flowing east). Li Yu was poisoned on his birthday shortly after writing this lyric, reportedly for its politically dangerous content.
usage
了 here reads liǎo (= to finish, end), not the perfective le. Inseparable from 往事知多少 as the opening couplet. 何时 = when (rhetorical). Quote for cyclical weariness, unbearable nostalgia, or to invoke the ghost of the Southern Tang.
