From the year when/in which he was going to school he began to know what he wanted when he grew up.

从他要上学的那年,他就开始意识到他想长大后成为什么.
from the year when / in which he was going to school 在句子中做时间状语,其中when / in which he was going to school 是the year的定语,可以用when或in which来引导,因为中心词是时间the year.
what he wanted when he grew up.从句在句子中做宾语.when he grew up 又做时间状语.
这个句子可以简化成: From the year, he began to know what he wanted.