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English Translation

lit.: the old man lost his horse, but it all turned out for the best
fig.: a blessing in disguise

Breaking it down

1. 塞: tactical border position, fortress
2. 翁: old man
3. 失: to lose
4. 马: horse
5. 焉知: lit. how is one to know?
6. 非: not
7. 福: blessing, good fortune

Source: Google Image Search

Story in Chinese

Classical Chinese

近塞上之人有善术者。马无故亡而入胡。人皆吊之，其父曰：“此何遽不为福乎？” 居数月，其马将胡骏马而归。人皆贺之，其父曰：“此何遽不能为祸乎？”家富良马，其子好骑，堕而折其髀（读音bì，股，大腿）。 人皆吊之，其父曰：“此何遽不为福乎？”居一年，胡人大入塞，丁壮者引弦而战，近塞之人，死者十九，此独以跛之故，父子相保。 故福之为祸，祸之为福，化不可极，深不可测也。——《淮南子·人间训》

Modern Chinese

在靠近边塞的人中，有一位精通术数的人。他家的马自己跑到胡人那里去了，大家都来慰问他。 这位父亲说：“为什么就知道不是福运呢？”过了几个月，他家的马带领着胡人的骏马回来了，大家都祝贺他。这位父亲说：“为什么就知道不是祸端呢？”家里有许多骏马，他的儿子喜欢骑马，有一次从马上摔下来折断了大腿骨。 大家都慰问他，这位父亲说：“为什么就知道不是福运呢？”过了一年，胡人大举侵入边塞，男子健壮的都拿起弓箭参战，塞上参战的人，十个死九个，不死的都是重伤。唯独他的儿子因为腿摔断了的缘故，父子得以保全性命。 所以福可变为祸，祸可变为福，这其中的变化难以捉摸，深不可测。

Source: baike.baidu.com

Story in English

There once were people living near a frontier fortress, and amongst them was a man who was proficient in numeracy. One time his horse ran away to the barbarians. Everyone came and wanted to comfort him.
But the father said: "Why do you all know that this is not good luck after all?"
After a few months, the horse returned and brought with him a fine horse from the barbarians. Everyone congratulated the father on this.
But he said: "Why do you know that this is not a disaster?"
There are many horses in the family, and the father's son liked to ride them. One time however, when riding a horse he fell off and broke his leg. Everyone tried to console him.
Again, the father said. "Why do you know that this is not a blessing?"
After a year, the barbarians came to invade the frontier fortress. All healthy men took up bow and arrow and went to war with the barbarians. Out of 10 men, 9 died, and those who didn't were seriously injured.
Only his son was spared because of his broken leg.
Therefore, blessings can be turned into misfortunes, and misfortunes can be turned into blessings. Such changes are hard to pin down and impossible to predict.

Translation by Celine Li

Usage

他丢了旧车，保险公司赔他一部新车，真可说是塞翁失马，焉知非福 。