一、代词it
用法例句代替前文提到过的东西或事情，用作人称代词This is a new dictionary. I bought it yesterday.
这是一本新词典，我昨天买的。
Tom joined the army last mouth. Do you know about it?
汤姆上月入伍了，你知道这事吗？
代替指示代词，起着this或that的作用Is this your car? No, it isn’t.
这是你的汽车吗？不是。
What’s that? —It’s a video.
那是什么？它是一台录像机。
Whose room is this? —It is theirs.
这是谁的房间？是他们的。
指明某人或某物的身分，还可指不明性别的婴儿Who’s there? It’s me. (It’s John)
谁在那儿？是我。（是约翰）
Go and see who it is that rings. It’s Bill.
去看看是谁来的电话，是比尔。
The Greens have a new baby. It’s lovely.
格林家有一个新生婴儿。它很可爱。
指时间，距离，自然现象（天气，气候，明暗）量度，价值等It’s Sunday today. 今天是星期天。
It’s three months since he came here.
自从他来到这里，已经三个月了。
How far is it to the Great Wall.
到长城有多远？
It’s getting dark. 天快黑了。
—How much is the coat? —It’s ninety dollars.
这件衣服多少钱？九十美元。
二、引导词it
用法例句形式主语 代替不定式短语It is up to you to decide. 这事要由你决定。
It makes me happy to hear you have recovered.
听说你恢复了健康，这使我很高兴。
代替动名词短语It’s no good smoking. 吸烟没有好处。
It’s worthwhile working the whole night on the problem.
用整夜的时间来研究那个问题是值得的。
代替主语从句It doesn’t matter what you do. 你干什么都没有关系。
It seems that everyone has known the news.
好象大家都知道这个消息。
形式宾语 代替不定式短语I consider it better to be early.
我认为能够早一些更好。
We found it impossible to get there before July .
我们觉得，要在七月以前到达那里是不可能的。
代替动名词短语We thought it no use doing that.
我们认为做那事没有用。
代替宾语从句The teacher makes it clear that everyone should hand in his homework on time.
老师清楚地指出，每个人都应该按时交作业。
三、it在强调结构中
强调句型：It is (was) 被强调部分 that (who) 其它
强调部分 例句主语 It is I who am to blame. 是我该受责怪。
It was your uncle that (who) came yesterday.
昨天来的是你叔叔。
宾语 It was a new pen that Mother gave me .
母亲给我的是一支新钢笔。
状语地点It was in the classroom that I left my umbrella.
我把雨伞就落在了教室里。
时间It is at eleven that the train leaves.
火车是在十一点钟离开。
方式It was just as he ordered that I acted.
我正是照他吩咐的那样做的。
原因It was because he was in trouble that I tried my best to help him.
正是因为他处境困难我才尽力帮助他。
宾补It was red that we painted the gate.
我们把门油漆成红的颜色。
It was chairman that they elected him .
他们选他担任的是主席的职务。
说明：原句的谓语动词如果是现在或将来时态用It is … that (who)如果原句谓语动词是过去时态，则用It was … that (who)。在强调时间，地点，原因或方式时不要用when, where, how, 必须用that。
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