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:
loveit.../dislikeit.../dependonit.../seetoit.../relyonit.../appreciateit./makeitclear.../takeitforgranted+宾语从句.如:
Weshouldmakeitclearwhenwecanleave.我们应该搞清楚何时可以离开.
Don'ttakeitforgrantedthatyoucanbeatthem.不要想当然你们会赢他们.
Iwillappreciateitifyoucanhelpme.如果你能帮我,我将不胜感激.