一、what不可以用在定语从句中，that或which引导定语从句。what在从句中一般引导名词性从句或感叹句
1、引导名词性从句
名词性从句缺少主语，宾语，表语，而缺少的这个词不是指具体的人时，首先考虑what。
如You can imagine what it would be like driving a car into a brick wall at 30 miles an hour.
你可以想象汽车以每小时30英里的速度撞向一堵砖墙结果会怎样。
2、引导感叹句
I can't believe what a beautiful girl she is.
what在从句中充当主语，宾语或表语，意思是“什么”或“所.........的”。
二、定语从句中关系代词用that，which，两者用法上的区别主要为：
1、that可以指人，也可以指物；which只能指物
2、当先行词是不定代词all, something, anything,everything, none, the one等时，只能用that如： Is there anything that I can do for you?
3、先行词前有形容词的最高修饰时，要用that。如： This is the best film that I have ever seen for years.
4、先行词前有序数词修饰时，要用that如： This is the fifth book that I have ever written.
5、先行词前有形容词only, very, few, little等修饰时，要用that如： She was the only person that was invited to the palace.
6、先行词既指人，又指物时，只能用that
扩展资料:
判断关系代词与关系副词:
一、用关系代词，还是关系副词完全取决于从句中的谓语动词。及物动词后面无宾语，就必须要 求用关系代词。
例如:(错)This is the mountain village where I visited last year.
(错) I will never forget the days when I spent in the countryside.
(对)This is the mountain village (which) I visited last year.
(对) I'll never forget the days (which) I spent in the countryside.
习惯上总把表地点或时间的名词与关系副词 where, when联系在一起。此两题错在关系词的误用上。
二、准确判断先行词在定语从句中的成分(主、谓、宾、定、状)，也能正确选择出关系代词/关系副词.
先行词在从句中作主、定、宾语时，应选择关系代词；先行词在从句中作状语时，应选 择关系副词。
例如：
（对）Is this the museum which you visited a few days ago?
（对）Is this the museum where the exhibition was held?