it is用法可以分为两种，一种it指物，是真正主语，例如It is a dog. 它是条小狗。还有种是把it看做形式主语，真正的主语是后面的不定式to do,例如It is difficult for us to finish the homework.我们很难完成家庭作业。句型为it is hard/difficult/possible...for sb to do sth.
（1）It is time（for sb.）to do sth./lt is（high）time that sb.did sth.（虚拟语气）该是做某事的时候了。例如：
It is high time that we tried our best.//It is time for us to try our best.
（2）It is+形容词（+of/for sb.）+to do sth. （3）It is+形容词+that+sb.+（should）do sth.（虚拟语气）能用于这个句型的形容词有
strange、wonderful、natural、good，proper、right、wrong、well、fortunat e、important、necessary、useless、likely、probable、impossible等。有时可省去should而直接用动词原形。如：
It is important that we should pay close attention to grain.
It is natural that he（should）say so. （4）It is no use/good+doing sth.做某事无意义 It is no use trying again =It is of no use to try
again.
（5）It is+被强调部分+that/who+.. Was it in the street that you met her？ Who was it that called him\"comrade”？
It was not until yesterday that I met my old friend. It is in a small factory that my brother is going to work.注意上述各句为强调句型的“考点”） 比较：It is a small factory where my brother is going to work.（定语从句）
（6）It is said/reported/announced/（well）known..…that...据说/据报道…
（7）It is/was+时间+since.…从..…已多久了。
It is/has been three years since I met him in Beijing. It was a long time since I（had）lived in that small mountain village with these farmers.
（8）Itis+时间+before..…
这个句型和上面句型中时间都是时间段，如
long、years、months、weeks、five days、three hours、twenty minutes等。
不过主句的时态多用一般将来时或一般过去时。 It will be several years before we meet again. 我们要过好几年才能再见面。
It was not long before they set out for the front.
不久他们就出发去了前线。
（9）It is/was/will be the first/second/..…time that....第几次做某事It is the first time that I have been here.
It was the second time that he had seen the film. （10）It is up to sb.to do sth...应由某人做某事。 It is up to you to decide whether we start or not.
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