前事不忘，後事之師

From The Intrigues of the Warring States:

臣觀成事，聞往古，天下之美同，臣主之權均之能美，未之有也。前事之不忘，後事之師。君若弗圖，則臣力不足。 [Traditional Chinese poetry, trad.]

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臣观成事，闻往古，天下之美同，臣主之权均之能美，未之有也。前事之不忘，后事之师。君若弗图，则臣力不足。 [Traditional Chinese poetry, simp.]

From: Zhanguo Ce, circa 5^th – 3^rd centuries BCE

Chén guān chéngshì, wén wǎnggǔ, tiānxià zhī měi tóng, chén zhǔ zhī quán jūn zhī néng měi, wèi zhī yǒu yě. Qiánshì zhī bùwàng, hòushì zhī shī. Jūn ruò fú tú, zé chén lì bùzú. [Pinyin]

I have done research on some successful cases and drawn lessons from events of ancient times and found that people of the world long for perfection. However, there has been no precedent that both sovereigns and high-ranking court officials can reach perfection in pursuing and maintaining their power. Lessons can always be drawn from previous events. If you don't think about it this way, there is nothing I can do about it.

Alternatively, from Jia Yi's famous prose Ten Crimes of Qin:

野諺曰：“前事之不忘，後事之師也。”是以君子為國，觀之上古，驗之當世，參以人事，察盛衰之理，審權勢之宜，去就有序，變化有時，故曠日長久而社稷安矣。 [Classical Chinese, trad.]

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野谚曰：“前事之不忘，后事之师也。”是以君子为国，观之上古，验之当世，参以人事，察盛衰之理，审权势之宜，去就有序，变化有时，故旷日长久而社稷安矣。 [Classical Chinese, simp.]

From: c. 170s BCE, 賈誼,《過秦論》下, translation based on Burton Watson's work

Yěyàn yuē: “Qiánshì zhī bù wàng, hòushì zhī shī yě.” Shìyǐ jūnzǐ wéiguó, guān zhī shànggǔ, yàn zhī dāngshì, cān yǐ rénshì, chá shèngshuāi zhī lǐ, shěn quánshì zhī yí, qùjiù yǒuxù, biànhuà yǒushí, gù kuàngrìchángjiǔ ér shèjì ān yǐ. [Pinyin]

A homely proverb goes, “Former affairs, not forgot, teach those who come after.” That is why, when a gentleman is given charge of a state, he observes how things were done in ancient times, tests them in terms of the present day, sees how they tally with human concerns, examines into the cause of flourishing and decay, perceives what is fitting in the light of circumstances, initiates actions in the proper order, and changes with the times. Thus, his days as sovereign are long and many, and his realm rest secure.

前事不忘，後事之師

1. don't forget past events as they can guide you in the future​; benefit from past experience