英语中后置定语如何判断用什么关系代词？什么时候只能用that，什么时候只能用which、who、whom之类？

Question

最好每种情况举下例子。
像I will never forget the day,on "which" I first saw him.
I'm sure there's nothing "that"our monitor can't do.
That book is the "that" he bought in biggest bookshop in our city.
为什么只有唯一的答案

Answer 1

I will never forget the day,on "which" I first saw him. 答案唯一，不用that，不能省

I'm sure there's nothing "that"our monitor can't do. 先行词是nothing、anything，everything等只用that，本题在从句作宾语所以可省

That book is the first/ last (one)"that" he bought in biggest bookshop in our city.有序数词或最高级只用that

定语从句有时答案不唯一
this is the factory in which/ where he works.

this is the factory that/ which produces cars.主语不能省

this is the factory that/ which/不填 he used to work in.
this is the factory, which produces cars. 非限制性定语从句，that不能填

请参考一下

Answer 2

用到关系代词就不是后置定语而是定语从句了。
当先行词也就是关系代词前面的那个名词，为各种不定代词，或被the,some,every 等词修饰时，只能用that.追问

那which和who呢

追答

当先行词是物时用which，which可做宾语，也可做主语。
当先行词是人时用who, 关系代词做主语用who，做宾语就用whom

Answer 3

定语从句的引导词分成关系代词和关系副词。介词+which=关系副词，看先行词在从句中作什么成分。