1、 It is no use (no good, fun, great fun, a new experience, a great honour, etc) doing sth
2、 It is/was+被强调的部分+that(who)+其他
3、3——it做形式主语、真正主语的三种形式；
4、② 与客观事实不符时，从句需使用虚拟语气，
5、 why从句 It was clear why he had asked for a conference. 他为什么要求召开会议原因很清楚。
6、 It is+时间段+since+主语+did
7、 a. That he has left here is clear. 他已离开了这儿这点很清楚。
8、 It is+时间段+before+主语+谓语
9、It sounds/looks+形容词，意为“听/看起来
10、 whether从句 It makes little difference whether we go or stay. 我们去还是留没有多大差别。
11、Itissuggested/ordered/commanded+that
12、 It is well-known+ that从句 “众所周知”
13、It is impossible/necessary/strange+that
14、It is/was said/reported+that从句，
15、③ It is Tom who/that told me about the
16、It is/was+表示时间的名词+when+从句
17、 用作连接代词表示“什么”(由疑问词变来)。如：
18、① 表示客观事实时，从句不用虚拟语气；
19、It takes sb.+时间+to do sth.
20、 It is good (nice, useless, interesting, pleasant, tiring, etc) doing sth
21、It is+形容词+for+sb.+to do sth.
22、③ 与现在事实不符，从句谓语动词用should do
23、② It was because he was ill that he didn’t
24、 It looks as if...意为“看起来……”
25、It happens that..意为“碰巧”
26、 when从句 It hasn't been made clear when the conference is to take place. 会议什么时候召开还没有宣布。
27、 a. That we'll hold a meeting is decided. 我们要召开一次会议这已决定了。
28、It is/was+表示地点的名词+where+从句
29、 It is+a pity/a shame+that从句
30、It seems that...意为“似乎…”
31、It 指代时间、距离、价值、天气，以及不明确的人
32、 It is time that+从句主语+should do/did+其