求一道语法题的解析Once they had fame,fortune,secure futures; now (all

求一道语法题的解析
Once they had fame,fortune,secure futures; now (all that is left) is utter poverty.请问括号里的all that is left为什么不是all what is left?语法上怎么讲?

Once they had fame,fortune,secure futures; now (all that is left) is utter poverty.请问括号里的all that is left为什么不是all what is left?语法上怎么讲?
答：句意：他们曾经有名望、有财富,有无虑的未来,而现在剩下的只是彻贫.
all是个代词,后跟定语从句,用that引导.what不是关系代词,不能引导定语从句.
比较：
The doctor did what he could (do) to save the girl's life.
那医生做了他所能做的（事情）来救那女孩的生命.
what:连接代词,引导名词性从句（宾语从句）,相当于：the things that-定语从句.
The doctor did all (that) he could (do) to save the girl's life.
那医生做了他所能做的一切来救那女孩的生命.
all是“一切”,作did的宾语,that引导一个定语从句,在从句中作省掉的那个do的宾语,所以可以省略.
all that is left 就是：所剩相的一切,that是关系代词,引导限制性定语从句,在从句中作主语,不能省略.
不过,all .is ...,译作：只是.
如：All we could see from the plane is the white clouds gathered beside us which stretches away as far as the eye could see