可以并存.
如：What he discribed was a computer which/that we didn’t know how to use at that time.
What he discribed 主语从句
was a computer系表结构
which/that we didn’t know how to use at that time.定语从句,修饰先行词a computer
因为我打拼音,把【主语从句】错写成了【状语从句】.