用作状语的从句叫状语从句。状语从句可置于句首、句末或句中，一般来说，状语从句都比较完整。但是，在一些状语从句中存在的省略问题，可以归纳为以下两点：
1. when, while, if, as if, although / though, as, until, once, whether, unless, where等引导的从句中，如果其谓语为be，而主语与主句的主语相同时，则从句的主语和be可省略。有时从句的主语为it时，也可省略it或从句中的有关成分。如：
①When (I am) in trouble, I always turn to my classmates for help. 遇到麻烦时，我总是找同学帮忙。
②If(it is) possible, this machine can be fixed at once.可能的话，这台机器会立即修好。
③When(water is) pure, water is a colourless liquid. 水清时，是无色的液体。
④When(it is) necessary, you can help us to do something.必要时，你可以帮我们做点儿事。
2. than, as, no matter what /who等后面成分的省略。如：
①We do not use more materials than (it is) necessary.我们不用过多的材料。
②We go by way of Beijing as (it was) originally planned.我们按原计划途经北京。
③He is much better today than (he was) when I wrote to you last year. 他今天比我去年给你写信时好多
1. —Did Jack come back early last night?
—Yes. It was not yet eight o’clock _______ he arrived home.
A. before B. when
C. that D. until
2. You should make it a rule to leave things _______ you can find them again. [NMET 1999]
A. that B. where
C. which D. what
3. Give children the space to voice their opinions, ______ they are different from your own.
A. until B. even if
C. unless D. as though
4. I always take something to read when I go to the doctor’s _____ I have to wait.
A. in case B. so that
C. in that D. as if
5. The old tower must be saved, ________ the cost.
A. however B. whatever
C. whichever D. wherever
6. It would be worth trying ______ you can not succeed at once.
A. unless B. whether
C. as though D. even though
7. This set of furniture can last for several generations ______ properly protected.
A. that B. although
C. if D. and
8. Unlike watching TV, reading is a highly active process ______ it requires attention as well as memory and imagination.
A. until B. but
C. unless D. for
Key: 1-4 BBBA 5-8 BDCD
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