墨守成规

mò shǒu chéng guī

to cling to old ways

Character by character

墨mò
Mozi

守shǒu
guard

成chéng
set

规guī
rules

时代变了，做事不能墨守成规。

Times have changed; we can't keep doing things the old way.

Tone: negative (criticism).

墨 is Mozi, famous for defending cities; here, rigidly defending set rules.

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