什么时候用what引导定语从句

应该是what引导的定语从句。例如：

1. What was impossible 50 years ago may be quite possible today .

   50年前不可能的事情今天可能是完全可能的。

2. She told them what she had seen there.

   她告诉他们她在那里看到了什么。

3. They are building a second library in what was once the school cinema.

   他们正在学校的电影院里建第二个图书馆。