it用作形式主语或形式宾语的用法
1. it用作形式主语
当不定式、动名词、从句等用作主语时，为避免头重脚轻，通常在句首使用形式主语it，而把真正的主语放在句子末尾。如：
It’s a pity to refuse. 拒绝是令人遗憾的。
It’s no use saying any more about it. 再谈这事没有用。
It’s not known where she went. 她到哪里去了没人知道。
注：It is said / reported / believed / understood that… 这类结构中的 it 也是形式主语。如：
It’s reported that two people were injured in the accident. 据报导这次事故中有两人受伤。
2. it用作形式宾语
当不定式、动名词、从句等用作宾语且其后跟有宾语补足语时，就应在宾语补足语前使用形式宾语，而将真正的宾语移至句末。如：
I find it easy to get on with her. 我发现她很容易相处。
I think it best that you should stay here. 我认为你最好住这儿(from www.lulutubu.com)。
They haven’t made it known where they are to hold the conference. 他们还没宣布会议在哪里开。
◎几种特殊的形式宾语it◎
1. enjoy, like, dislike, love, hate, prefer, appreciate等动词后接if 从句或when 从句，通常应先在动词后接it 作形式宾语。如(from www.lulutubu.com)：
She won’t like it if you arrive late. 她不喜欢你迟到。
He hates it when people use his bike. 他讨厌别人用他的自行车。
I hate it if you say such things in public. 我讨厌你在大庭广众之下说那样的事。
I’d prefer it if I didn’t have to do so much work. 要是我不必做那么多工作，那就太好了。
I would appreciate it very much if you would help me with it. 如果你能帮助我做这事，我会十分感激。