教训 (jiàoxun) is a noun meaning "lesson" or "moral," specifically a lesson learned from a mistake or negative experience. It frequently appears in the phrase 吸取教训 (to learn a lesson), and can also be used as a verb meaning "to teach someone a lesson."
这次失败是一个深刻的教训。
Zhè cì shībài shì yī gè shēnkè de jiàoxun.
This failure was a profound lesson.
我们一定要吸取教训,不能再犯同样的错。
Wǒmen yīdìng yào xīqǔ jiàoxun, bù néng zài fàn tóngyàng de cuò.
We must learn from this lesson and not make the same mistake again.
这件事给了所有人一个教训。
Zhè jiàn shì gěi le suǒyǒu rén yī gè jiàoxun.
This incident taught everyone a lesson.