有读者问： 有一本英语语法书上说，当关系代词用作主语补语时和当定语从句为there be句型时，都只能用that。这样说对吗？
这种说法不妥。根据近年英国出版的一部具有权威性的英语语法巨著《英语语法大全》（A Comprehensive Grammar of the English Language ）（17.25），关系代词在限制性或非限制性定语从句中用作非介词的补语时，都只能用which。这里所谓的补语即你所说的主语补语，也就是我国所通用的表语。实例如：
(1)She is the perfect accountant which her predecessor was not.
（限制性定语从句，which指人的特征）
(2)This is not the type of modern house which my own is.
（限制性定语从句，which指物）
(3)He is teetotaller, which I am not.
（非限制性定语从句，which指人的特征）
(4)This is a powerful car, which my last car was not.
（非限制性定语从句，which指物）
但上述规则似乎有点绝对化了。实际上，关系代词that亦可用作表语，但一般皆用于限制性定语从句中，如：
(5)He died like the hero that he was.
(6)It has taken China several decades to make it the beautiful city that it is today.
这种用作表语的that也可省去，如：
(7)I suppose he's altered from the boy he was.（the boy之后省去了that）
(8)If you behave like the modest unassuming young things you are, you couldn't fail to be a success anywhere. （things之后省去了that）
下面两句中的who其实也用作表语：
(9)The trouble is that people don't believe that I'm who I say I am.
(10)Isn't that who you are?
用作定语的there be结构，开首倒确是多用that（常可省去），但that在此似乎更应说是主语，如：
(11)It is the best dictionary （that ）there is.
(12)We are trying hard to narrow the gap （that ）there is between town and country.
(13)I have told you all （that ）there is to tell.
但偶尔which也可用作上述定语从句的表语，如：
(14)But all right, Mr. Poggioli, suppose there is a La Fronde, which there isn't.
