It is +…… 后面什么时候加doing ， 什么时候加to do 啊？

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。

有些是固定搭配，如It is no use /good doing
It is adj.+to do sth

it is time for + doing
it it time to + do