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English Translation

lit.: to mend the fold after the sheep is lost
fig.: to act belatedly, better late than never

Breaking it down

1. 亡: to lose
2. 羊: sheep
3. 补: to mend, to repair
4. 牢: fold (for animals)
5. 为时未晚: it is not too late

Source: Google Image Search

Story in Chinese

战国时楚襄王荒淫无度，执迷不悟，将劝谏的大臣庄辛赶出楚国。秦国趁机征伐，很快占领楚都郢。楚襄王后悔不已，派人到赵国请回庄辛，庄辛说：“见兔而顾犬，未为晚也；亡羊则补牢，未为迟也。”鼓励楚襄王励精图治、重整旗鼓。

Source: baike.baidu.com

Story in English

During the Warring States period, the king of Chu, called Xiangxiang was persistent in going about matters in a destructive manner, and so he drove out his minister Zhuang Xin. The state of Qin took this opportunity and captured the capital of the state of Chu. The king of Chu then regretted his decision and sent someone to the country of Zhao to invite his former minister Zhuang Xin back.
The minister then said: "It is not too late to mend the fold after the sheep are lost, better late than never" So he encouraged the king of Chu to do his best to regroup his troops.

Translation by Celine Li

Usage

桌子虽然被虫咬了，但只要及时补，就能做到亡羊补牢。
虽然电脑中了病毒，不过现在安杀毒软件，也是亡羊补牢, 为时未晚。