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it既是高中英语教学的难点，也是近几年高考的热点。熟练使用it，对提高书面表达能力，培养英语语感都会有很大的帮助。下面就其使用的重点、难点，分门别类加以总结。
一、用于强调结构
1.“It is＋被强调部分＋that．．．”是强调句型，即将被强调的部分放在“it is”后面，将其它部分置于“that”后面。被强调的部分可以是句子的主语、宾语、宾语补足语或状语。如果被强调的主语是人，可用who替代that。如果将“It is”和“that”去掉，再把各种句子成分重新排列，还应该是一个完整无缺的句子。这也是辨别强调句式与其它从句的有效方法。例如：
It was about 600 years ago that the first clock with a face and an hour hand was made.
It was they that(who)cleaned the classroom yesterday.
I was in the street that I met her father.
2.“It is not unitl十被强调部分十that．．．”也是强调句型，主要用来强调时间状语，可译为“直到……才……”。例如：
It was not until she took off her dark glasses that I realized she was a famous film star.=Not until she took off her dark glasses did I realize she was a famous film star.=I didn't realize she was a famous film star until she took off her dark glasses.
二、作形式主语或形式宾语，代替后面作真正主语或真正宾语用的短语或从句
1.作形式主语用于含主语从句的主从复合句
①在“It is clear(obvious,true,possible,certain．．．)that．．．”句式中，It是形式主语，真正的主语是that引导的主语从句。例如：
It is very clear that the elephant's leg is round and tall like a tree.
=That the elephant's leg is round and tall like a tree is very clear.
②在“It is important(necessary,right,strange,natural)that．．．”句式中，由于主句中形容词意义的影响，that从句中的谓语动词应为“should＋动词原形”，且should可以省去。例如：
It is important that we(should)learn English well.
It is necessary that he(should)remember these words.
③在“It is said(reported,learned,believed．．．)that．．．”句式中，It仍是形式主语，真正主语是that引导的主语从句。该结构常被译为“据说（据报道，据悉……）”。例如：
It is said that he has come to Beijing.
It is reported that another earth satellite has been put into orbit.
④在“It is suggested(ordered,decided,advised．．．)that．．．”句式中，主句中的过去分词是表示请求、建议、命令的词，that从句的谓语动词要用“should＋动词原形”，其中的should也可以省去，常译为“据建议；有命令；有决定……”。例如：
It is suggested that the meeting(should)be put off.
It was ordered that we(should)arrive there in two hours.
⑤在“It is a pity(a shame．．．)that．．．”句式中，that从句中的谓语动词一般用“should＋动词原形”，其中的should也可省去，表示出乎意料，常译为“……真遗憾，（……真可耻）”。如果没有这种意义，则不用上述动词形式。例如：
It is a pity that such a thing(should)happen in your class.这种事发生在你们班上，真是遗憾！
It is a pity that he is ill.可惜的是他生病了。
⑥在“It happens(seems,looks,appears)that．．．”句式中，It是形式主语，that引导的从句是主语，主句中的happen,seem等是不及物动词。例如：
It happened(so happened)that he met his teacher in the street.
It seems that he will be back in a few days.
⑦在“It doesn't matter whether(if)．．．”句式中，whether(if)引导的从句是真正的主语，常译为“是否……”。例如：
It doesn't matter if they are old.
2.作形式主语代替用作真正主语的短语
①在“It takes sb．．．to do sth”句式中，不定式是真正的主语，It是形式主语，常译为“做……要花费某人……”。例如：
It took thousands of people many years to build the Great Wall.
②在“It is no good(use)doing sth.”句式中，真正的主语是动名词短语，主句中的表语可以是no good(not any good),no use(not any use)。例如：
It is no good learning English without speaking it.
③在“It is kind(of sb)to do sth”句式中，不定式短语是真正的主语。如果不定式的逻辑主语是由of引起，主句中的表语必须是能表示逻辑主语特征的形容词。常见的这类形容词有：bad,brave,careless,clever,cruel,foolish,good（好心的）,honest,horrible,kind,lazy,modest,naughty,nice（有教养的），polite,rude,silly,stupid,wise,wrong等。这个句式可以改写为：Sb.
is＋上述形容词＋to do sth.例如：
It is kind of you to say so.=You are kind to say so.
④在“It is necessary(for sb)to do sth”句式中，如果不定式的逻辑主语是由for引起，主句中的表语通常是表示重要性、紧迫性、频繁程度、难易，安全等的形容词，常见的这类形容词有：important,necessary,natural,easy,hard,difficult,dangerous,impossible等。例如：
It is important for her to come to the party.=It is important that she(should)come to the party.
3.作形式宾语代替用作真正宾语的短语或从句
在“主语＋think/believe/feel/make/consider＋it＋形容词／名词＋真正宾语”句式中，it是形式宾语。例如：
We think it our duty to clean our classroom every day.
He felt it useful learning English well.
They found it difficult that they would finish their work in two days.
I felt it a surprise that they were all unfriendly to me.
He thinks it necessary that you will help him.
三、用于“It is time that．．．”等句式中
①在“It is time(about time,high time)that．．．”句式中，that从句中常用一般过去时态，有时也用“should＋动词原形”，但should不能省略，常译为“是（正是）……的时候了”。例如：
It is time that children should go to bed.=It is time that children went to bed.
②在“It is the first(second．．．)time that．．．”句式中，that从句中的谓语动词不用虚拟语气，而用完成时态。如果主句中的谓语动词是一般现在时，则从句中的谓语动词为现在完成时；如果主句中的谓语动词为一般过去时，则从句中的谓语动词为过去完成时，常译为“是第一（二）……次……”。例如：
It is the first time that I have been here.=This is the first time that I have been here.
四、用于“It is．．．since/when．．．”句式中
①在“It is．．．since．．．”句式中，主句中以时间段作表语，谓语动词时态是一般现在时或现在完成时，since从句中谓语动词通常是瞬间动词的一般过去时态；如从句中的谓语动词是延续性动词，则该时间段的起点是从动作结束时算起。例如：
It is(has been)five years since his father came here.他父亲来这儿已经5年了。
It is(has been)five years since his father left here.他父亲离开这儿已经5年了。
②在“It is．．．when．．．”句式中，when引导的是一个时间状语从句，主句中的It指时间，表语由表示具体时间点的词语来充当。例如：
It was five o'clock when he came here.
③在“It be．．．before．．．”句式中，主句中的It指时间，主句中谓语动词的时态是一般将来时或一般过去时。主句中的表语多是long,not long,three days,two weeks等表示时间段的词语。例如：
It was three days before he went to Beijing.3天后，他去了北京。
It will be not long before he finishes his job.不久，他就把他的工作做完了。
五、用于其他情况
1.在“It looks(seems)as if．．．”句式中，It无意义，as if引导一个表语从句。常译为“看起来好像……”。如果与事实不相符合，则谓语动词用虚拟语气。例如：
It looks as if he is ill.（真的病了）
It looks as if he were ill,（没有生病）
It seemed as if he were dying.
2.it指代“心目中的人”或“上下文中提到的人或事物”。例如：
①—Who is it?
—It's me,Lin Wen.
②That's it!（就这么回事！）
③He is fifty-two,but he doesn't look it.（他52岁了，但看上去不像那么大。）
④Whatever you do and say,ask yourself whether it is in the interests of the people.
⑤Did you see my pen?I put it on the desk this morning.
3.用作无人称代词it，指代时间、气候、距离等。例如：
It's fine today.
It's five o'clock now.
It's a five minutes' walk from my home to the school.
4.在性别不详或性别开关紧要时，可用it指动物、幼童等。例如：
I threw the lillte dog to him and he caught it.
They got a baby and it was a ten-pounder.（他们添了一个10磅重的婴儿）