It is +adj./n. + for sb. to do sth. 做某事对某人来说……
It是形式主语，真正的主语是不定式to do sth. for sb. 是不定式的逻辑主语。例如：
It's not easy for us to study English well. 对我们来说学好英语并不容易。
It's a good idea for us to travel to the south. 去南方旅行对我们来说是个好主意。
对我来说把英语学好非常重要。to learn English well.
It's + adj. + of sb. to do sth.
It是形式主语，to do sth.是真正的主语， of sb. 是逻辑主语，当表语(即形容词)能对逻辑主语描述时，常用介词of，而不用for.例如：It's very polite of you to give your seat to old people. 你给老人让座，非常有礼貌。 It’s very kind of you to help me.
你能来车站接我真是太好了。to pick me up at the station.
It seems/appears that… (在某人看来)好像……
此句中的it是主语，that引导的是表语从句。例如：It seems that he is lying. 看样子他好像是在撒谎。It appears to me that he never smiles.
看样子要下雨了。it's going to rain.
It is +数词+metres/kilometers long/wide… ……是多少米(公里)长(宽)
用来表示物体的长(宽，高)，如数词大于一，名词要用复数。例如：
It is 20 metres long from this end to that end. 从这端到那端有二十米长。
It's time for sb. to do sth. 是某人干某事的时候了
it是形式主语，真正的主语是动词不定式to do sth.。for sb./sth.是逻辑主语。例如：
It's time for the child to go to bed. 孩子该睡觉了。
比较下面两种结构：① It's time for + n. 例如： It’s time for school.
②It's time to do sth. 例如： It’s time to go to school. 我们该学习英语了。
It takes sb. some time to do sth. 花费某人多少时间做某事
it是形式主语，真正的主语是动词不定式to do sth.。例如：
It takes her fifteen minutes to walk to the bus stop from here. 从这儿走着到公交车站将花费她15分钟。
It took the old man three days to finish the work. 那个老人花了三天时间完成这项工作。
我上学坐公交车要花半个小时。
keep (on)doing sth. 一直坚持做某事
keep doing sth.一般用于静态动词。keep on doing sth.意为“继续不停地做某事”，一般用于动态动词，但二者的区别并不是很严格，有时可以互换。例如：
Don't keep on doing such foolish things. 不要再做这样的傻事了。
He kept sitting there all day. 他整天坐在那里。
keep…from doing sth. 阻止，使免于做某事
相当于stop…from doing sth. prevent…from doing sth.。在主动句中，stop和prevent后面的from可以省略，但在被动结构中，from不可以省略。例如：
Please keep the children from swimming in the sea. 请别让孩子到海里游泳。
The big noise outside my room stopped me from doing my homework. 屋外巨大的噪音使我不能做作业。
我会尽最大努力阻止他抽烟。I'll try my best to
keep…from doing sth. 阻止，使免于做某事
相当于stop…from doing sth. prevent…from doing sth.。在主动句中，stop和prevent后面的from可以省略，但在被动结构中，from不可以省略。例如：
Please keep the children from swimming in the sea. 请别让孩子到海里游泳。
The big noise outside my room stopped me from doing my homework. 屋外巨大的噪音使我不能做作业。
我会尽最大努力阻止他抽烟。I'll try my best to .
keep sb. doing sth. 让某人一直做某事
不可和keep sb.from doing sth.结构混淆。
例如：Why do you keep me waiting for a long time? 你为什么让我等了很长时间?
