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。