定语从句中，如果从句修饰的是一个语义或短句，该用什么先行词

which/ as 都可以。
which :1.只能放在主句之后，
2， 没有汉语意思，
as: 1.放在主句之前或后。
2。有汉语意思，“正如”
如：He didn't pass the exam, which made his parents very angry.
先行词是真个句子：He didn't pass the exam， 关系代词用which.
As we all know, the earth is round.

which

用as，不用which 和 that
例如， He is good at English, as is known to all.
He can play basketball very well, as I admired .