what 是主句的宾语,在从句中也是句子成分,有实际意义,that一般做定语
比较一个,i am going to do the thing that i want to do.那么。he is the only student who understand English well .中的who能不能改成that呢。为什么:-!可以，that既可以指人也可以指物，指人时相当于who