到底神马时候用that 和what (同位语从句中)

题目

到底神马时候用that 和what (同位语从句中)
对比:) I have no idea what has happened to her .
(2) Suddenly the thought came to him that he could go blind.
这两句中what 和that 可以互换吗?

答案

不能替换,（1）句中what在同位语从句中作主语（从句缺了主语）
（2）句中that只起连接作用（因为从句中不缺成分,也没有疑问）