一、what不可以用在定語從句中，that或which引導定語從句。what在從句中一般引導名詞性從句或感嘆句
1、引導名詞性從句
名詞性從句缺少主語，賓語，表語，而缺少的這個詞不是指具體的人時，首先考慮what。
如You can imagine what it would be like driving a car into a brick wall at 30 miles an hour.
你可以想象汽車以每小時30英里的速度撞向一堵磚牆結果會怎樣。
2、引導感嘆句
I can"t believe what a beautiful girl she is.
what在從句中充當主語，賓語或表語，意思是“什麼”或“所.........的”。
二、定語從句中關係代詞用that，which，兩者用法上的區別主要為：
1、that可以指人，也可以指物；which只能指物
2、當先行詞是不定代詞all, something, anything,everything, none, the one等時，只能用that如： Is there anything that I can do for you?
3、先行詞前有形容詞的最高修飾時，要用that。如： This is the best film that I have ever seen for years.
4、先行詞前有序數詞修飾時，要用that如： This is the fifth book that I have ever written.
5、先行詞前有形容詞only, very, few, little等修飾時，要用that如： She was the only person that was invited to the palace.
6、先行詞既指人，又指物時，只能用that
一、what不可以用在定語從句中，that或which引導定語從句。what在從句中一般引導名詞性從句或感嘆句
1、引導名詞性從句
名詞性從句缺少主語，賓語，表語，而缺少的這個詞不是指具體的人時，首先考慮what。
如You can imagine what it would be like driving a car into a brick wall at 30 miles an hour.
你可以想象汽車以每小時30英里的速度撞向一堵磚牆結果會怎樣。
2、引導感嘆句
I can"t believe what a beautiful girl she is.
what在從句中充當主語，賓語或表語，意思是“什麼”或“所.........的”。
二、定語從句中關係代詞用that，which，兩者用法上的區別主要為：
1、that可以指人，也可以指物；which只能指物
2、當先行詞是不定代詞all, something, anything,everything, none, the one等時，只能用that如： Is there anything that I can do for you?
3、先行詞前有形容詞的最高修飾時，要用that。如： This is the best film that I have ever seen for years.
4、先行詞前有序數詞修飾時，要用that如： This is the fifth book that I have ever written.
5、先行詞前有形容詞only, very, few, little等修飾時，要用that如： She was the only person that was invited to the palace.
6、先行詞既指人，又指物時，只能用that