it是形式宾语,when引导的就是宾语从句,但你的句子中少了谓语动词.可用于这种句式的还有：
love it.../ dislike it.../depend on it.../see to it.../rely on it.../appreciate it./make it clear.../take it for granted +宾语从句.如：
We should make it clear when we can leave.我们应该搞清楚何时可以离开.
Don't take it forgranted that you can beat them.不要想当然你们会赢他们.
I will appreciate it if you can help me.如果你能帮我,我将不胜感激.