在
复合句中用作定语的从句称为
定语从句。定语从句通常位于所修饰的名词或代词之后，常由
关系代词或关系副词引出，被修饰的词称为
先行词。常见的关系代词有：who\whom\which\that\whose,关系副词有：when\why\where.
一、先行词为指人的名词或代词时，定语从句用who或that来引导。例如：Do you know the girl who is in a red dress.你认识那个穿红衣服的女孩吗？
The man who we saw that day hasn't come here yet.那天我们见到的那个人还没有来这儿。
二、先行词是指物的名词或代词时，用which或that, which或that在从句中做主语或宾语。例如：She works in a shoe factory which\that is far from here.她在离这儿很远的一家鞋厂工作。
三、只能用关系代词that引导的几种情况：A、当先行词是anything,nothing,everything等
不定代词时。例如：I'm sorry to tell you that there is nothing that I can do for you.很抱歉告诉你，我帮不上忙。
Is there anything that you want to buy in town?你在城里有什么东西要买吗？
B、当先行词是最高级或被最高级修饰时。例如：This is the best way that can solve the problem.这是解决问题的最好方法。
This is the most beautiful flower that I have seen.这是我看到的最美的花。
C、当先行词被first,last,only,very,no,just,any,little等词修饰时。例如：She asked for the last book that was on the subject.她要了有关这个科目的最后一本书。
This is the only one that I'm looking for.这是我正在找的唯一的一个。
四、who引导的定语从句
A、who引导的定语从句的先行词为表示人的名词或代词。例如：The boy who is standing under the tree is Peter.
B、who在定语从句中作主语时不能省略；但在非正式的英语中，who也可作宾语，且可以省略。例如：The person who was here yesterday is a teacher.
The man (who) I saw just now is Mr Liu.
C、在定语从句中，who在人称、数上和其前面的先行词保持一致。例如：Do you know the boy who is standing over there?
D、若先行词中既有人又有物，关系代词用that，而不用who。例如：He watched the children and boxes that filled the car.他看见了塞满汽车的孩子和盒子。
E、若先行词前面有形容词最高级、
序数词修饰，关系代词用that而不用who。例如：Yao Ming is the best basketball player that I kown.
五、当先行词为地点名词时，用关系副词where引导。例如：This is the place where we came last summer.
六、当先行词为表示时间的名词时，用关系副词when来引导。例如：She came on the day when I was not at home.她来的那天我不在家。
这些都是我上课做的笔记，唔，码字好辛苦的~~手都酸了，不过对我自己也有帮助的呢，我也复习了一遍……(*^__^*) 嘻嘻……希望对你有所帮助。
啊，还有，你问的问题我觉得that可以换成which，不能用what。 嗯，我是学生，建议你问问你的老师……