句型1

It (so) happened（chanced） that +clause. = sb. happened /chanced to do sth. =sb.did sth. by chance. 如： 
It happened that he was out when I got there. 当我到那儿时，碰巧他不在。＝He happened to be out when I got there.= It chanced that he was out when I got there= He was out by chance when I got there. 

句型2
It seems that sb. do/ be doing/ have done/ had done= Sb. seems to do/ be doing/ have done/to be done/to have been done（还有动词appear等可这样使用）如:

It seemed that he had been to Beijing before.他好象以前去过北京。＝He seemed to have been to Beijing before. 

句型3
It is / was＋被强调的部分＋that(who)+剩余的部分.如： 
It wasn't until he came back that I went to bed.直到他回来我才睡觉。(一定要注意被强调句型中的谓语动词否定的转移)。 It was because he was ill that he didn't come to school today.只因为他有病了今天没有来上学。（只能用because而不能用for, as 或since） 
It is I who am a student. 我确实是个学生。（句中am不能用are来代替。） 

句型4
It is high time (time/ about time)+ (that) 主语＋should do / did+其它。（从句中的谓语动词用的是虚拟语气。）如: 

It is high time that we should go / went home.我们该回家了。 

句型5
It is / was said ( reported…)＋that+从句. 如： 
It was said that he had read this novel.据说他读过这篇小说。＝He was said to have read this novel. 

句型6
It is impossible / necessary/ strange…that clause.(从句中的谓语用should＋do / should have done，其形式是虚拟语气。)如： 
It is strange that he should have failed in this exam.真奇怪，他这次考试没有及格。 

句型7
It is + a pity/ a shame…that clause.(注意从句中的谓语动词用should do或should have done的形式，但should可以省略。)如： 
He didn't come back until the film ended. It was a pity that he should have missed this film. 他直到电影结束才回来。他没有看到这部电影真可惜。