指人时有时只用 who 不宜用 that。
1．先行词为 one、ones 或 anyone
Anyone who breaks the law should be punished.
2．先行词为 these 时
These who are going to Beijing are the best students of our school.
3．在 there be 开头的句子中
There is a student who wants to see you.
4．一个句子中带有两个定语从句，其中一个定语从句的关系代词是 that，另一个宜用who，以免重复。
The student that won the first prize is the monitor who works hards.
5．在非限制性定语从句中
A。I met a friend of mine in the street, who had just come from Japan.
B．主句以 who 开头的句子中,只用 that,不用 who。
指物时，下列情况下只能用 which，不宜用 that。
（1）关系代词放在介词之后
This is the factory in which we once worked.
（2）非限制性定语从句中
This is the book, which is written by Lu Xun.
（3）that，Those 作主语时
Those which are on the desk are English books.
E．先行词前有 such、the same、 as 时, 关系代词用 as，不用 that, 但 the same…as…表示同样的, the same…that…表示同一的
He knows as many people as are present at the meeting.
Such people as you refer to are rare nowadays.
限制性定语从句与非限制性定语从句的区别：
限制性定语从句
非限制性定语从句
All the students who study hard have passed.学习努力的学生都考试及格了。(只有努力的及格了，不努力的没及格。) All the students ,who study hard have passed.所有的学生都及格了，他们学习努力。（没有人不及格，这些学生都很努力。）
从句是先行词不可缺少的定语（如去掉，主句意思就不完整或不正确。）
从句是对先行词的附加说明（如去掉，主句意思仍清楚，不受影响。）
与主句关系密切，不用逗号 与主句关系不十分密切，用逗号与主句隔开译为汉语时，从句译在先行词前，与先行词用“的”连接 译为汉语时，从句放在主句后面关系代词 that 可以代替 who、whom(指人)、which(指物) 关系代词不用 that,只用 who、whom(指人)、which(指物) 关系代词代替宾语时可以省略 关系代词不能省略关系代词 as 和 which
先研究下面两个例句：
○1.This elephant is like a snake, as /which everybody can see. =As everybody can see, this elephant is like a snake. 任何人都看得出来，这头象像条蛇。
○2.Tom didn’t pass the physics exam, which made his parents very angry. Tom 物理考试不及格，这使得他父亲很生气。
这两个例句中，as 和 which 所代表的都是整个主句所表示的内容。但有两点不同之处：
1． 在形式上 as 引导的非限制性定语从句可位于主句的后面，也可位于主句的前面；而 which 引导的非限制性定语从句只能位于主句的后面，不能位于主句的前面。
2． 在意义上，as 引导的定语从句和主句的关系一般为一致关系，常译为“正如……”，“就像……”，而 which 引导的定语从句和主句的关系是因果关系，或 which 引导的定语从句是对主句的评论。因此，在意思通顺的情况下，which 可代替 as，而 as 许多时候不能代替 which，如句○2。再如：
1．He was late again, as / which we had expected. =As we had expected, he was late again. 正如我们所料，他又迟到了。/他又迟到了这是我们早就料到的。
2． The street hasn’t been cleared for weeks, which makes it very dirty. 街道好多星期没有打扫了，因此整条街很脏。（不用 as）
3．The young man cheated his friend of much money, which was disgraceful. 那年轻人诈骗了他朋友许多钱财，这是不光彩的。（不 用 as）
4．He takes exercise everyday, which has done a lot of good to his health. 他每天运动，这对他的身体很有好处。（不用 as）
5．As has already been pointed out, English is rather difficult for a foreigner. 正如已经指出的那样，英语对于初学者说，是相当难学的。在 such…as…、the same…as…、 as…、 as many…/much as…等结构中,as 不能用 which 代替。如：
1.Such books as this are too difficult for beginners. 这样的书对于初学的人来说是太难了。
=Books such as this are …
=Books like this are …
2.I live in the same building as he (does). 我和他住再同一座大楼里。
3.He knows as many people as are present at the meeting. 他认识所有到会的人。
4.That day we all got up early as usual. 那天我们和平时一样，都起得很早。
“One of the +复数名词”后面定语从句中谓语单复数情况
这一结构后面的定语从句的谓语动词通常用复数形式，跟定语从句所*近的那个复数名词在数上保持一致。如：
1． That is one of the books that are required for study at school.
2． This is one of the most wonderful novels that have been published since 1990.
3． She is one of the few persons who know Spanish .
4． This is one of the most famous plays that were written in the thirties,
如果 one of +复数名词这一结构前面带有 the only、the very 之类的限定语，后面定语从句的谓语动词则要用单数形式，这是因为定语从句在意义上修饰的是而不是那个复数名词。如：
1.He is the only one of the teachers who knows French in our school.( 修饰the only one)
他是我们学校懂法语 的唯一一个老师。
He is one of the teachers who know French in our school.( 修饰 the teachers)
他是我们学校（众多）懂法语的 的老师中的一个。
2.This is the only one of the rooms that is free now. (修饰 the only one)
This is one of the rooms that are free now. (修饰 rooms)
as 与 which 引导的定语从句
两者均可引导非限制性定语从句，有时可以互换，但下列情况多用 as。
1. 关系代词引导的定语从句居句首时。
As we all know, the earth is round.
2. 当与 such 或 the same 连用时，一般用as。
Such books as you tell me are interesting. I have the same plan as you.
3. 当从句和主句语义一致时，用 as，反之则用which。
She has married again, as was expected. She has married again, which was unexpected.
4. as 在从句中作主语时，后面常接行为动词的被动语态，如 be known，be said，be reported 等，如从句中行为动词是主动语态，一般要用 which 作主语。
She has been late again, as was expected.
Tom has made great progress, which made us happy.