惩前毖后
chéng​qián​bì​hòu

to learn from past mistakes

  1. 惩 懲 chéng
    to learn a lesson
    (literary) to take warning from a past mistake; to restrain oneself; to regret or repent
  2. 前 qián prepositionlocative
    before
    (bound form) ago; before; earlier; previously
  3. 毖 bì
    careful
    (bound form) careful; cautious; to guard against
  4. 后 後 hòu locativepreposition
    after
    (bound form) later in time; afterwards; future; subsequent in a sequence