11. It is …when…

该句型中的when引导的是一个时间状语从句，主句中的it指时间，表语由具体的时间充当。常译为“当……的时候，是……”
It was 5 o’clock when he came here.

12. It be…before…

该句型主句中的it指时间，主句中的时态常是一般将来时或过去时两种时态，主句中的表语多是long ,not long ,3 days, 2 weeks等表示时间段的词和短语。常译为“…之后才”，“没过……就……”
It was 3 days before he went to Beijing.
It will not be long before he finishes his job.

13. It happens (seems, looks, appears) that…

该句型中的it 是形式主语，that 引导的是主语从句，主句中的happen, seem等词是不及物动词。
It happened (so happened) that he met his teacher in the street.  (碰巧……)
It seems that he will be back in a few days.  (看来……)
The Foreign Minister said, “___our hope that the two sides will work towards peace.” (2004北京)                        
A. This is   B. There is   C. That is   D. It is (D)

14. It takes sb…to do sth.

该句型中的不定式是真正的主语，是形式主语，句型中的直接宾语是时间。常译为“做……要花费某人……”
It took thousands of people many years to build the Great Wall.

15．It is no good (use) doing sth.

该句型中的真正主语是动名词短语，主句中的表语可以是 no good (not any good), no use (not any use ).
It is no good learning English without speaking English.

16. It doesn’t matter whether…

该句型中的引导的从句是真正的主语，该句型译为“不论（是否）……没关系……
It doesn’t matter whether they are old.

17．It is kind (of sb) to do sth.

该句型中的不定式短语是真正主语，如果不定式的逻辑主语是由of引出，主句中的形容词必须是能表示逻辑主语的形容词。常见的有：bad, brave, careless, clever, cruel, foolish good (好心的),honest, horrible, kind, lazy, modest, naughty, nice (有教养的),polite, rude, silly, stupid ,wise ,wrong等。这个句型可以改写为sb.is kind to do sth.
It is kind of you to stay so =You are kind to say so.

18. It is necessary (for sb) to do sth

该句型与上一句统属一个句型。如果不定式的逻辑主语是由for 引出的，主句的形容词通常是表示重要性，紧迫性，频繁程度，难易，安全等情况的形容词。常见的形容词有：
Important,necessary,natural,easy,safe,common,,normal,hard,difficult,dangerous,unusual,grape, impossible, pleasant.等。
It is important for her to come to the party. =It is important that she (should) come to the party.

19. It looks (seems) as if …

该句型中it 无意义。As if 引导一个状语从句，常译为：“看起来好像……”如果与事实不相符合，则用虚拟语气。
It looks as if he is ill. （真的病了）
It looks as if he was ill. （事实上没有生病）
It seemed as if he was dying.

20. We think it important to learn a foreign language.

该句型中的it作形式宾语。为了记忆方便我们可称该句型为“7123结构”
7指主句中的常用的动词：think, believe, make, find, consider, feel, take.
1指的是形式宾语it .
2指的是宾补的两种形式：形容词和名词
3指的是真正宾语的三种形式：不定式短语，动名词短语或that引导的宾语从句。
We think it our duty to clean our classroom every day.     
XiaoLi felt it important learning English well.
They found it difficult that they would finish their work in two days.
-Do you like ____here?
-Oh, yes .The air, the weather, the way of life. Everything is so nice. (2004 全国卷二)
A. this B. these C. that D. it    (D)
分析：it作形式宾语