“what is crucial to a successful classroom is to involve students.”the professor said.（be crucial to）
“一个成功课堂的关键是让学生参与进来。”那位教授说
【1】划线部分为什么使用what做主语
解答：“What”为“融合型连接代词”或者“融合型关系代词”引导主语从句，此时what=the thing that.复合型连接代词what引导主语从句，并在主语主语从句作主语。
what is crucial to a successful classroom【主语】 //is 【系动词】//to involve students【表语】//.
=The thing that is crucial to a successful classroom is to involve students.
【2】what is crucial to 可以使用Being crucial to代替吗？
解答：what is crucial to 不可以用Being crucial to代替。
【3】to involve students语法软件说这部分是状语后置，那请问老师它修饰的是前面的哪个部分。
解答：不定式to involve students为不定式作表语。
