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
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.
1.It is/has been+一段时间+since... "自从...已有多久"
It has been two weeks since he left
2.It was+一段时间+before... "在...之前过了多长时间"
It was three years before he had finished his research
3.It will be+一段时间+before... "过了多久才..."
It will be two days before she arrives at Britain