1。强调结构:It is/was ... that ....
例句:It was at 10 o'clock that I heard the news.
2。表示时间(非强调结构)
例如:It was 10 o'clock when I heard the news.
3。表示“现在离(某事发生)已经有(一段时间)了。”
/“那时离(。。发生)已经有(一段时间)了。”
It is (some time) since .... = It has been (some time) since ....
It was (some time) before ....= It had been (some time) before....
例如:It is 8 years since China joined WTO.
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