it is polite words which will ...为什么要把which改成that

it is polite words which will ...为什么要把which改成that

关系代词在定语从句中作表语时,只用that.（表语一般就是跟在be动词后面的词）
拓展一下：
在下列情况关系代词只能用that而不用which.
先行词为all,everything,anything,nothing,none,little,few,much等不定代词时.
eg.You must do everything that I do.
2) 先行词被all,every,no,any,little,much,some,just等修饰时.
eg.Is there any question that troubles you much
3) 当先行词被序数词或形容词最高级以及the only,the very,the last等修饰时.
eg.James is the best singer that I have ever known.
4) 当先行词是人和物构成的词组时.
eg.They were talking about the people and places
( that ) they had visited.
5) 当句中有who 或which 时,为避免重复.
eg.Which is the book ( that ) he borrowed from the library
eg.Who is the person that is talking to our headmaster?
6).关系代词在从句中作表语时;
eg.My hometown is no longer the village that it used to be.追答polite words即是表语，that是关系代词，在句子中that代替polite words也充当表语成分。这是定语从句，that就是代替polite words的，但是不能省略