语法精讲
it的基本用法
（1）用作人称代词，代替前文提到过的事物。The train has arrived. It arrived half an hour ago.
（2）用以代替提示代词this, that。
—What’s this? —It’s a knife. —Whose watch is that? —It’s mine.
（3）起指示代词的作用，指一个人或事物。—Who is knocking at the door? —It’s me.
（4）指环境情况等 It was very noisy (quiet) at the very moment.
（5）指时间、季节等。 —What time is it? —It’s eight o’clock.
（6）指距离。 It is a long way to the school.
（7）作形式主语。It is not easy to finish the work in two days.
（8）作形式宾语。I think it no use arguing with him.
（9）用于强调结构。It was Xiaoming whom(that)I met in the street last night.
含有“It is …”的句型
（1）It is time (for sb.) to do sth. It is (high) time that sb. did sth.(虚拟语气)
（2）It is + 形容词（+of / for sb.）+ to do sth.
通常用of的词有brave、clever、careful、hopeless、kind、good、naughty、nice、silly、stupid、foolish、wise等。
（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、fortunate、important、necessary、useless、likely、probable、impossible等。有时可省去should而直接用动词原形。如：It is natural that he（should）say so.
（4）It is no use/ good + doing sth.
It is no use trying again = It is of no use to try again.
（5）It is + 被强调部分 + that/who + …
It was not until yesterday that I met my old friend.
It is in a small factory that my brother is going to work.
（6）It is said/reported/announced/ (well) known … that…
（7）It is/was + 时间 + since … 从……已多久了。
It is five months since I arrived in New York.我到纽约已经五个月了。
It is five months since I was in New York.我离开纽约已经五个月了。
（注意：终止性动词表示从动作发生时算起；如果是状态动词或持续性动词，则从动作或状态结束时算起。）
（8）It is + 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/third … time that …
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.