定语从句引导词只用that不用which

楼主的问题很专业，其实关于which到底能不能做表语，很多权威语法书都有争议。目前为止，是第三种说法正确。当先行词在主句中做表语，或者，引导词在从句中作表语，符合任一种情况都只能用that。Shenzhen is no longer the city that it used to be .属于引导词在从句中作表语。This isn't the book that I borrowed from the library.属于先行词在主句中作表语。

有争议说which可作表语，但也只有这样的例子，She was fond of dancing, which her brother never was.只能说明是在非限制性定语从句中,which 可作表语。