关系代词who， whom， whose， which， that是用用引导定语从句的：
1. who/whom/that
三者都可指代人: who作主语， whom 作宾语， that既可以作主语， 又可作宾语。
如:
he is a good teacher who/that is loved by all.
here is the man (whom) you are looking for.
2. whose 可修饰人，也可修饰物 修饰物时， 可与of which换用，whose在定语从句中作定语。
如:
I saw some trees whose leaves were black with diseases.
= I saw some trees the leaves of which were black with diseases.
3. that/which/that/which指代物， 在定语从句中可作主语或宾语。如:
emei is a wonderful place which/that is worth visiting.
the film (which/that) we saw last night is wonderful.
疑问代词用于特殊疑问句中，疑问代词一般放在句子的最前面，在句中可用作主语、宾语、表语、定语。
whose sweaters are these? (定语)
what class are you in， tom? (定语)
whom are you looking for? 你在找谁? (宾格)
what is that? 那是什么?(表语)
who was here just now? 刚才谁来过这儿? (主格)
(一般说来who在句子中作主格，whom在句子中作宾格)
which one do you like， this one or that one? 你喜欢哪一个，这个还是那个?(定语
