It is + no use, no good (fun, a great pleasure, a waste of time, a bore...)等名词+doing sth.
It is no use crying.哭没有用。It is no good objecting.反对也没有用。
It is a great fun playing football．打篮球很有趣。
It is a waste of time trying to explain．设法解释是浪费时间。
It is + useless (nice, good，interesting, expensive等形容词)+ doing sth．
It is useless speaking.光说没用。
It is nice seeing you again．真高兴又遇到了你。
It is good playing chess after supper．晚饭后弈棋挺好。
It is expensive running this car．开这种小车是浪费。
引导词it的用法
（1）作形式主语
代替由
不定式（或不定式复合结构）、
动名词或从句所表示的真正主语。
eg: It was wrong for you not to help her.
你当时不帮助她是错误的。(it代替不定式短语)
It will be useless learning a theory without practice.
学习理论而没有实践是无用的。(it 代替动名词短语)
It’s not yet known where she has gone.
她去哪里了还不知道。(it代替
主语从句)
（2）作形式宾语
代替由不定式、动名词或从句所表示的真正宾语。 eg:
We consider it necessary to master two foreign languages.
我们认为掌握两门外语是有必要的。(it作动词不定式的
先行词)
We found it useless talking to her.
我们认为跟她谈是无用的。(it作动名词的先行词)
I must make it clear that I’ll always support you.
我必须申明我将永远支持你。(it作that引出的
宾语从句的先行词)
（3）用在强调结构中
为了强调句子的某一成分（通常是主语、宾语或状语），常用“It is(was) +被强调成分+that（或who）……”结构。 eg:
I met Tom in the street yesterday. 我昨天在街上遇见了汤姆。
It was I who met Tom in the street yesterday..是我昨天在街上遇见汤姆的。
It was Tom that I met in the street yesterday.昨天我在街上遇见的是汤姆。
It was in the street that I met Tom yesterday.我昨天是在街上遇见汤姆的。
It was yesterday that I met Tom in the street.我是昨天在街上遇见汤姆的。
注：如果去掉“It is (was) ……”这一结构，将被强调部分放回原句，其句子仍然成立。 eg:
Where is it that we shall park our car?我们把车停在哪？
（去掉is it that，原句还原成:Where shall we park our car?）
3.几种易混句型：
(1) ①It was four years since I had left the school.我毕业4年了。
②It was four years that the war lasted.战争持续了4年。
注：①since引导时间状语，不能用that替代；
②
强调句型，强调了时间状语。
(2) ①Was it 1949 when our town was liberated?
我们镇解放的时间是1949年吗？
②Was it in 1949 that our town was liberated?
我们镇是在1949年解放的吗？
注：①It指时间，when在从句中作状语，不能由that充当；
②强调句型。
(3) ①It is (about, high) time that you closed your store.
到你关闭商店的时候了。
②It is the first time that I have been to the Great Wall.
这是我第一次去长城。
注：①
虚拟语气句型，从句谓语动词要用
过去式表示对现在情况的虚拟；
②that习惯用法，不能用when代替。
(4) ①It is a pity that Tom has lost the game.很遗憾，汤姆比赛失败了。
②It is Tom that has lost the game.是汤姆比赛失败了。
注：①It作形式主语；②It引导词，强调句型。