has it been announced when the plane is to take off中为什么用when

题目

has it been announced when the plane is to take off中为什么用when

答案

这句话的意思是通知飞机起飞的时间了吗?
when是对it的说明,是一个主语从句,代表起飞的时间,所以就要用when.
it被通知,这个it是什么呢,后面就用when来说明了,是起飞的时间.