① 先行词是不定代词few;all;something;anything;everything;none等时，关系代词只能用that。
All that he said is true.他所说的全都是真的。
He did everything that he could to help us.他做了一切能够做的事来帮助我们
② 当先行词被all;every;no;just;the very;the only;the last等修饰时，关系代词只能用that。
All the things that we can do are to give you some advice.我们能做的事就是给你一些建议。
This is the very dictionary that I want. the very在句中表“正是…”
这正是我想要的那本字典。
③ 当先行词被序数词或形容词最高级修饰，或先行词本身就是充数词或形容词最高级时，关系代词只能用that.
The first thing that my younger brother is going to do this afternoon is to buy some pencils. 今天下午我弟弟要做的第一件事是去买一些铅笔（The first在句中表序数词修饰名词）
I have found the best way that could finish the exam.我已经找到了完成测试的最佳办法了。
（4）当先行词既包含人又包含物时，关系代词只能用that。
We talked about the people and things that we remembered.
(5)当主句以who或which开头的特殊疑问句时关系代词只能用that。
Who is the woman that is standing at the gate?