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　　根据it的用法特点可归纳为２０个句型。
　　1. It is + 被强调部分 + that …
　　该句型是强调句型。将被强调的部分放在前面，其它部分置于that之后。被强调部分可以是主语，宾语，表语 或状语。强调的主语如果是人，that可以由who换用。如果把这种句型结构划掉后，应该是一个完整无缺的句子。这也是判断强调句型与其它从句的方法。
　　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.
　　It was in the street that I met her father.
　　2. It is not until + 被强调部分 + that …
　　该句型也是强调句型。主要用于强凋时间状语，译成汉语“直到…才…”，可以说是 not … until … 的强调形式。
　　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.
　　3. It is clear （ obvious, true, possible, certain…） that ……
　　该句型中it 是形式主语，真正的主语是that 引导的主语从句，常译为“清楚（显然，真的，肯定…）”　是主语从句最常见的一种结构。
　　It is very clear that he's round and tall like a tree.
　　= That he's round and tall like a tree is very clear.
　　4. It is important （ necessary, right, strange, natural…） that …
　　该句型和上一个同属一个句型。由于主句中的形容词不同，that 后的从句中要用虚拟语气（should + 动词原形），should 可以省去。建议记住该句型中的形容词。
　　It is important that we （should） learn English well.
　　It is necessary that he （should） remember these words.
　　5. It is said （reported, learned…） that …
　　该句型中的it 仍是形式主语，真正主语是that 引导的主语从句。该结构常译为“据说（据报道，据悉…）”。
　　It is said that he has come to Beijing.
　　It is reported that another earth satellite has been put into orbit.
　　6. It is suggested （ ordered … ） that …
　　该句型和上一个同属一个句型。主句中的过去分词是表示请求，建议，命令等词时，that　后的从句要用虚拟语气（should + 动词原形），should 可以省。常译为“据建议；有命令…）
　　It is suggested that the meeting （ should ） be put off.
　　It was ordered that we （ should ） arrive there in two hours.
　　7. It is a pity （ a shame … ） that …
　　该句型中，that后的从句一般用虚拟语气（should + 动词原形），should可省去。表示出乎意料，常译为”竟然“。没有这种意义时，则不用虚拟语气。
　　It is a pity that such a thing （ should ） happen in your class.这种事竟然发生在你们班上，真是遗憾！
　　It is a pity that he is ill. 他生病了，真遗憾！
　　8. It is time （ about time ,high time ） that …
　　该句型中that 后的从句应该用虚拟语气，值得注意的是① 常用过去时态表示虚拟。② 有时也用should + 动词原形，should 不能省。常译为”是（正是）…的时侯…“。
　　It is time that children should go to bed. = It is time that children went to bed.
　　9. It is the first （ second … ） time that …
　　该句型要和上一个句型区别开来。该句型中的 that 从句不用虚拟语气，而用完成时态。至于用什么完成时态，由主句的谓语动词的时态决定。如果是一般现在时，后面从句用现在完成时态；如果是一般过去时，后面从句则用过去完成时态。该结构中 that 可以省去；it有时用 this 替换。常译为”是第一（二）…次…“。
　　It is the first time I have been here. = This is the first time I have been here.
　　10. It is … since …
　　该句型主要用作处理瞬间动词的完成时，又要和表示一段时间的时间状语连用的问题。主句中是时间作表语，其时态是现在时或完成时，since 引导的从句通常是一般过去时态，而且是瞬间动词。如果主句是一般过去时，从句则用过去完成时。
　　It is （ has been ） 5 years since his father died.
　　11. It is … when …
　　该句型中的 when 引导的是一个时间状语从句，主句中的 it 指时间，表语由具体的时间充当。常译为”当…的时候，是…“。
　　It was 5 o'clock when he came here.
　　12. It be … before …
　　该句型主句中的 it 指时间， 主句中的时态常是将来一般时或过去时两种时态。主句中的表语多是long, not long , 3 days , 2 weeks 等表示时间段的词或短语。常译为”…之后…“。
　　It was 3 days before he went to Beijing.
　　It will be not long before he finishes his job.
　　13. It happens （seems, looks, appears ） that…
　　该句型中it是形式主语，that 引导的是主语从句，主句中的happen , seem等词是不及物动词。
　　It happened （ so happened ） that he met his teacher in the street. 碰巧
　　It seems that he will be back in a few days. 看来…
　　14. It takes sb. … to do sth.
　　该句型中的不定式是真正的主语，it是形式主语，句型中的直接宾语是时间。常译为”做…要花费某人…“。
　　It took thousands of people many years to build the Great Wall.
　　15. It is no good （use ） doing sth.
　　该句型中的真正主语是动名词短语，主句中的表语可以是no good , （ not any good ） , no use , （ not any use ）。
　　It is no good learning English without speaking English.
　　16. It doesn't matter whether （ if ） …
　　该句型中whether（if） 引导的从句是真正主语，该句型常译为　”不论（是否）…没关系…
　　It doesn't matter if they are old.
　　17. It is kind （ of sb. ） to do sth.
　　该句型中的不定式短语是真正主语，如果不定式的逻辑主语是由 of引起，主句中的形容词必须是能表示逻辑主语特征的形容词。常见的有：
　　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（错误的）等。 这个句型可以改写为：sb. is kind to do sth.
　　It is kind of you to say so. = You are kind to say so.
　　18. It is necessary （ for sb. ） to do sth.
　　该句型与上一个同属一个句型。如果不定式的逻辑主语是由for引起，主句中的形容词通常是表示重要性，紧迫性，频繁程度，难易，安全等情况的形容词。常见的形容词有：① important, necessary, natural ② easy , safe , common , normal , hard , difficult , dangerous , unusual, rare , impossible , pleasant等。
　　在①中的形容词作表语可以用从句改写， 如：
　　It is important for her to come to the party. = It is important that she （should ） come to the party
　　19. It looks （ seems ） as if …
　　该句型中it无意义。 as if 引导一个状语从句。常译为，“看起来好象…”如果与事实不相符合，则用虚拟语气。
　　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.
　　20. We think it important to learn a foreign language.
　　该句型中的it 作形式宾语。为了记忆方便我们可称该句型为“6123结构”。6指主句中常用的动词：think, believe, make, find, consider, feel；1指的是形式宾语it; 2指的是宾补的两种形式：形容词或名词；3指的是真正宾语的三种形式：不定式短语，动名词短语或that引导的宾语从句。
　　We think it our duty to clean our classroom every day.
　　He felt it important learning English well.
　　They found it difficult that they would finish their work in two days.