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在时间状语从句中，连词before除表示“在……之前”外，还常用来表示两件事情的发生相隔时间之长或之短。
Before I
could say anything he had read my mind. 我还未能说什麽，他就猜出了我的心思。
The host and
the hostess had waited for hours before the guest came. 主人等了几个小时以后客人才来了。
考点突破之重点精讲一“状语从句的省略”
Unless _____
to the meeting you should stay at home with your family.
A. invited
B. inviting C. being invited D. having invited
答案：A. 因为主从句的主语都是you将条件状语从句中的主语you和助动词are省略了。
1在时间状语从句（when while once until等引导的）条件状语从句，让步状语从句（thoughalthough等引导的），比较状语从句（than引导的）和方式状语从句中，如果主从句的主语相同，且从句中有be动词时，可以省去从句中的主语和be动词。例如：
Although he
is very old. He is still very strong 可省略成“Although very old. He is still very strong”
The product
is better than it is commonly supposed. 可省略成“The product is better than commonly supposed.”
2注意：如果状语从句中的主语是it且谓语为be动词时，即使主从句的主语不一致，也可以省略状语从句中的主语it和谓语be动词。例如：
I will come
early to help you prepare the speech if (it is) possible. 如果可能的话，我会早点来帮你准备演讲。
考点突破之重点精讲二“状语从句中的时态呼应”
--What are
planning to do in the future? ---No idea. After all I still have three terms to
go before I ______ .
A. will
graduate B. will have graduate C. graduate D. an graduating
答案：C. 在时间状语从句中要用一般现在时代替一般将来时。
1在时间，条件状语从句中常用一般现在时代替一般将来时，但有时也可在条件句中出现will表意愿。如：
If you will
come this wayour manager wants to see you. 请您这边来，经理想见你。
2在时间，条件状语从句中，也常用现在完成时代替将来完成时。在间接引语中的时间和条件状语从句中，常用一般过去时代替过去将来时，过去完成时代替过去将来完成时。例如：
The tuorists
will get to the destination in time if their car doesn’t break down.
如果车不出故障那些游客会及时到达目的地的。
I need one
more stamp before my collection is completed. 在我的集邮完整之前我还需要另外一枚邮票。
The
traveller won’t leave the hotel(招待所)until he has done all his business.
直到处理事务以后那个旅客才会离开旅馆。
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