Nanjing, from which the famous writer comes, is a big city.
这里的from which是comes from Nanjing的含义，选用from是因为come from。which指先行词Nanjing.
He stood on the top of the mountain, from where he could get a good view of the whole city.
这里的from where是指from on the top of the mountain。下面这句话可能会帮助你理解这种用法：
A cat ran out from under the desk. 一只猫从桌子底下跑走了。
from when的用法类似于from where
He graduated in 2000, from when he has been a teacher.
from when相当于from 2000
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