请讲一讲比较状语从句的特点。
比较状语从句通常由关联词 as 或 than 来引导。当由 as 来引导从句时，在主句中有副词 as 或 so 与之呼应。在肯定句中，as 从句表示“等同”比较，在否定句中则表示“不等同”比较。
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The bread was as hard as a brick. 这面包像砖头一样硬。
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You hate him as much as I( = as I hate him). 你跟我一样恨他。
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I can't run as/so fast as he can. 我不能像他跑得那么快。
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Isabelle has as many books as her brother (has). 伊沙贝尔拥有的书跟她哥哥一样多。
[附注]现代英语语法学家们已普遍认为,当主句为否定句时,既可以使用 not so...as,也可以使用 not as...as(见 not as...as)。
在正式语体中还有一种常见的句型: as/just as...so。位于句首的 as/just as 从句可以看作是比较状语从句。这种句型用于表示类推或比喻(参见 just as...so):
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As you hope to be treated by others, so you must treat them. 你希望别人怎样对你，你就得怎样对待别人。
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(Just) As a moth is attracted by a light, so he was fascinated by her. 就像飞蛾被光吸引一样，他被她吸引住了。
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Just as French people enjoy their wine, so the British enjoy their beer. 英国人喜欢本国的啤酒正如法国人喜欢本国的葡萄酒一样。
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Just as oil and water do not mix, so two people of such different characters cannot get on with each other. 正像油和水不能混合在一起一样,性格如此不同的人也是不能彼此相处的。
than 从句则表示“不等同”比较,在主句中有 more 或 less 或 -er 形式的形容词或副词的比较级与之相呼应:
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You speak Spanish much better than she does. 您说西班牙语比她说得好。
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He works less than he used to. 他比以前工作得少。
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Jane is healthier than her sister(is). 简比她姐姐健康。
clauses of concession (让步状语从句) 见 adverbial of concession
clauses of condition (条件状语从句) 见 adverbial of condition