前事不忘,后事之师

qián shì bù wàng , hòu shì zhī shī

don't forget past events, they can guide you in future (idiom); benefit from past experience

qián

front

shì

matter

no; not so

wàng

to forget

,

hòu

back

shì

matter

zhī

(possessive particle, literary equivalent of 的[de5])

shī

teacher