(Haigun no shou wa hei wo katarazu;
“The defeated commander speaks not of war.”)
Someone who has failed at something has no authority to speak on the topic; a defeated general should not give their opinions on warfare. Often used specifically to mean that one shouldn’t be a sore loser, or go around making excuses or casting blame after a failure; just own it and carry on.
We begin with the noun 敗軍 (haigun, pronounced “hi, goon!”), “defeated army.” The associative particle の (no) shows that this noun is attached to and modifying the noun 将 (shou), “commander,” which in turn is shown by the particle は (wa) to be the topic of discussion.
The comment on this topic centers on the verb 語る (kataru), “to speak of,” “to narrate [a story],” in imperfective form and taking the negative suffix ず (zu) in conclusive form. The particle を (wo) marks as its direct object the noun 兵 (hei), “warfare,” “strategy.”
It’s worth paying attention to the nuance here; I think there’s value in dissecting one’s own mistakes and failures in order to extract lessons from them. There’s a line between analysis and excuse-making, though, and another between that and straight-up whining, and it’s better not to cross either of those.
Variants warn that a defeated general should 謀らず (hakarazu), “not lay plans,” or 以て勇を言うべからず (motte yuu wo iubekarazu), “(with that,) must not speak of courage.”
Although the character 兵 can mean “soldier” in some contexts, reading it as such in this case is an error.
This saying is yet another selection from our friend, the Records of the Grand Historian. (Japanese 『史記』 = Shiki).
(“Yonrou no itoko ga nyuushi no hi no tenki nado ni tsuite yowane wo haki ni kita kedo, boku wa haigun no shou wa hei wo katarazu to itte, kiku no wo kyohi shita. Hontou ni uttoushii yatsu da.”)
[“My cousin, a four-year ronin, came to whine about how bad the weather was on the day of the entrance exam, but I refused to listen. I was all, ‘a general who lost shouldn’t talk about strategy.’ What a pain in the butt.”]