close the stable door after the horse has bolted
US /kloʊz ðə ˈsteɪbl dɔːr ˈæftər ðə hɔːrs hæz ˈboʊltɪd/
UK /kloʊz ðə ˈsteɪbl dɔːr ˈæftər ðə hɔːrs hæz ˈboʊltɪd/

1.
亡羊補牢, 事後諸葛亮
to take precautions after a misfortune has already happened
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Trying to fix the security breach now is like closing the stable door after the horse has bolted.
現在才試圖修復安全漏洞,就像是亡羊補牢。
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They should have implemented stricter rules earlier, but it's too late now; they're just closing the stable door after the horse has bolted.
他們本應早些實施更嚴格的規定,但現在為時已晚;他們只是在亡羊補牢。