一、It 作形式主语
1、It is + 名词/形用词/过去分词 + that 从句 (主从) = that 从句 (主从) is +名词/形用词/过去分词
It is well known that + 句子(众所周知)
It is obvious that + 句子 (明显的)
It is apparent that + 句子 (显然的)
例句：It is well known that diamonds are the hardest substance.
例句：It is universally acknowledged that trees are indispensable to us
全世界都知道树木对我们是不可或缺的.
例句：It is common knowledge that honesty is the best policy.
2. It is + 形容词/过去分词/名词 (for sb.)+ to do …
例句： It is hard (for me)to finish the work in three days.
It is an honor to meet you.
3.It was said/reported/ thought/believed + that从句(主从) 据说/报道/认为…
Sb. was said/reported/ thought to do …
例句：It was said that he had been killed in the accident.
此句子可以转化为：He was said to have been killed in the accident.
4. It took/ will take sb. + time to do … 某人花费时间干…
例句：It took me 10 days to finish my paper.
二、It 表示时间的常用句型
1.It is/has been+一段时间+since... "自从...已有多久"
since后的从句用一般过去时
It has been two weeks since he left
他已经离开了两个星期了
2.It was+一段时间+before... "在...之前过了多长时间"
before 后的从句用过去完成时
It was three years before he had finished his research
在他完成他的研究之前已经过去三年了
3.It will be+一段时间+before... "过了多久才..."
before后的从句用一般现在时
It will be two days before she arrives at Britain
两天后她才会到达英国
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