【考点点拨】
在复合句中，用作状语的从句叫做状语从句。状语从句有下列几种类型：
注意：在含有时间或条件状语从句的复合句中，如果主句用一般将来时，从句通常用一般现在时表示将来。
if引导的条件状语从句if意为“如果”，由if引导的条件状语从句，可以放在主句后，也可以放在主句前；当从句放在主句前时，常用逗号与主句隔开。条件状语从句表示主句动作发生的前提、条件等。
在含if引导的条件状语从句的复合句中，从句的时态通常遵循以下规则：
◆当主句为一般将来时态时，从句用一般现在时。如：
I will go with you if it doesn’t rain tomorrow.
◆当主句的谓语含有can, may, must等情态动词时，从句用一般现在时。如：
You can watch TV if you finish your home-work.
◆当主句是祈使句时，从句用一般现在时。如：
Don’t forget to turn off the lights if you leave
the office.
温馨提示：“unless 肯定句”相当于“if 否定句”。如：
You will fail unless you study hard. = You will fail if you don’t study hard.
until, although, so that引导的状语从句在复合句中，用作状语的从句叫状语从句。until引导时间状语从句，用于肯定句时，意为“直到……为止”，主句中的谓语动词用延续性动词；用于否定句时，意为“直到……才……”，主句中的谓语动词用非延续性动词。
although引导让步状语从句，意为“尽管；虽然”,通常可以与though互换使用，但二者都不能与but同时出现在一个句子中。
so that引导目的状语从句，从句中的谓语动词一般和can, could, may, might等情态动词连用。
unless, as soon as, so…that引导的状语从句◆unless意为“除非；如果不”，引导条件状语从句，常常可以和if … not句型互换。如：
I won’t let you in unless you show me your pass. = I won’t let you in if you don’t show me your pass. 如果你不出示通行证，我不会让你进去。
◆as soon as意为“一……就……”，引导时间状语从句，表示从句的动作一发生，主句的动作马上发生，强调动作的紧密性。如：I will call you back as soon as I arrive. 我一到就给你回电话。
◆so … that …意为“如此……以至于……”，so … that引导结果状语从句，so作副词，后接形容词或副词，that后接从句。如：The wind was so strong that we could hardly move forward. 风太大了，我们简直寸步难行。
She spoke so quietly that I could hardly hear her. 她说话轻得我几乎听不见。
【经典习题】1. —Your English is so good. How long have you been in England?
— I was six.
A. Until
B. Since
C. When
D. Before
2. —Would you like to go swimming with us tomorrow?
—Of course, I am busy.
A. unless
B. if
C. although
D. Because
3. The old lady gave lots of money to the poor students, she is not rich.
A. when
B. as soon as
C. if
D. Though
4. we don’t protect the earth, we will have no clean water to drink.
A. Until
B. Though
C. If
D. Before
5. —Where was your brother at this time yesterday?
—He was reading a book I was watching TV at home.
A. as soon as
B. while
C. until
D. after
KEY:
1-5 BADCB
