4.不過當先行詞是all的時候,定語從句通常用that引導,如果all是主句主語,那么all that兩個單詞可以用what代替,不過what引導的叫主語從句；如果all在主語中作賓語,剛換成what剛引導的叫賓語從句。

　　5.但是that和what是不可能同時引導定語從句的。

　　如：All that he taught me was very useful.= What he taught me was very useful.前句中的that引導定語從句,all為先行詞,而后句中的what引導的叫主語從句,what在主語從句中作taught的直接賓語。