When the plane is to take off has not been announced 飞机何时起飞还没有宣布
问题描述：
When the plane is to take off has not been announced 飞机何时起飞还没有宣布
为什么这边要使用 to take off又不能做谓语
答
When the plane is to take off 句子的主语 这个是一个主语从句 is to 表示 将来 be to do 表将要发生的事情