21. It is time when…
我到电影院时已经八点钟了。
It was 8 o'clock when I got to the cinema.
我离开医院时已经是半夜了。
It was midnight when I left the hospital.
22. It is time before…
不久我们就会再见面的。
I won't be long before we can meet again.
两年之后我们才能知道它是否有效。
It will be two years before we know whether it is effective.
23. It is…that…
我最珍视的是友谊。
It is friendship that I value most.
直到许多小工厂关闭了，这条河才又变得清澈起来。
It was not until the small factories were shut down that the river became clean again.
24. It is n / adj that / to do…
每个人都必须懂得如何使用计算机
It is a must that everybody should know how to use computers.
能够使用电脑对我们很方便。
It is convenient for us to be able to use computers.
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