1. 指物时，宜用that的情况（6点）
①当先行词是不定代词是，比如all, few, little, much, every, something, anything, everything, nothing, none, no, one, any, some等，关系代词只能用that。
Is there anything that I can do for you?
All the people that are present burst into tears.
②当先行词是有序数词，形容词最高级或the last, the only修饰时。
This is the most interesting film that I have ever seen.
The Yangtze river is the second longest river that exists in the world.
③当先行词有very, right, only, same修饰时，表示“仅仅”，“就是”，“正是”等。
This is the right/very book that I am looking for.
This pen is the only tool that you can use here.
④当先行词既指人又指物的时候。
They are talking about the persons and things that they met in Shanghai.
⑤避免重复。who和which开头的，关系词避免重复用that。
Who is the lady that is speaking French there?
Which is the pencil that you used just now?
⑥在there be的句型中或当先行词或者关系代词做从句表语时，常用that。
There are two novels that I want to read.
Tom is not the person that he used to be.
2. 指物时，宜用which的情况（6点）
①介词前置指物必须用which。
②在非限定性定语从句中指物，必须用which。
My borrowed pen, which is John's, is missing.
③在非限定性定语从句中修饰整个句子。
He failed in the test, which made her teacher very unhappy.
④当关系代词后面带有插入语时。
Here is the English Grammar which, as I have told you, will help improve your English.
⑤避免重复。当先行词为that，或者一个句子中两个定语从句，其中一个用了that，另一个宜用which。
I bought some magazines that might help me to kill time and which I could pass on to others when I finished them.
⑥当指人的身份特征时。
He is cheerful, which is his finest trait.
3. 指人时，宜用who的情况（4点）
①非限定性定语从句中指人只能用who。
②避免重复。如先行词为that/those时。
③当先行词是one, ones, anyone, anybody, everyone, everybody, someone, somebody时，关系代词应为who或whom。
④There be结构中，修饰主语的定语从句宜用who。
There's a gentleman who wants to see you.
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