it做形式主语后接that从句 为什么有的时候是连接代（副）词引导的从句.
比如 It was not clear to me why he behaved like that.
因为【连接副词和连接代词】都可以【引导主语从句】.
It was not clear to me why he behaved like that.= Why he behaved like that was not clear to me.
又如：
1.When they will come hasn’t been made pubic.
他们什么时候来还不知道.
2.Whether she is coming or not doesn’t matter too much.
她来不来都无关紧要.
3.What you need is more practice.
你所需要的是更多的训练.
4.Whatever we do is to serve the people.
我们无论做什么都是为人民服务.
如果带主语从句的句子是【疑问式】,则【必须用it作形式主语的结构】.
例如：
Has【it】 been settled who will be sent to carry out the task
谁将被派去执行这项任务决定了吗