HSK 7-9
马后炮
mǎhòupào名
(Chinese chess) moving the cannon after moving the horse away; (fig.) a useless action or comment; sth done after it's too late
事情结束后再批评，只是马后炮。
Shìqing jiéshù hòu zài pīpíng, zhǐshì mǎhòupào.
Criticizing only after the matter is over is just hindsight.
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Appears across levels (7-9) in the HSK 3.0 word list.