He who will not learn when he is young will regret it when he is old.
少壮不努力,老大徒伤悲.
本句格言的主句子是“He will regret it.”
“who will not learn”是对He做修饰的主语从句,而“when he is young”是对“will not learn”做修饰的状语从句,表明他什么时候不学习.
“when he is old"是修饰“regret it”的状语从句,表明他什么时候会后悔.