Than引导的比较状语从句我们都接触过，但than的用法却并不简单，有时候一些句子容易造成理解上的困难，比如上面截图中的两句：
(1) There ought to be less anxiety over the risk of mountain climbing than exists in the public mind.
(2) The swimming pool seems to be a great deal more luxurious than is necessary.
在两个句子中than都是作为连词引导比较状语从句，不过句子中出现了省略现象，因此可能会对语法分析造成困难，句子补充完整后是：
(1) There ought to be less anxiety over the risk of mountain climbing than (the anxiety that) exists in the public mind.
(2) The swimming pool seems to be a great deal more luxurious than (what) is necessary.
省略现象在than引导的比较状语从句中很常见，这里分析一下常见的几种省略类型。
than从句中有时候主语可以省略，在理解时我们要根据句子语境对主语进行补充，比如新概念英语3里面有这样一个句子：
To compensate for his unpleasant experiences in hospital, the man drank a little more than was good for him.
（为了补偿住院这一段不愉快的经历，那人喝得稍微多了一点。）
不少同学无法理解…the man drank a little more than was good for him这一用法，这里其实是省略了主语what，补充完整是：…the man drank a little more than (what) was good for him.
一些语法学家认为，than后面带what结构是累赘的表达，因此在比较正式的表达中，than后面的主语what经常省略。
文章开头出现的句子：There ought to be less anxiety over the risk of mountain climbing than (the anxiety that) exists in the public mind. 也是省略主语的情况。
在比较状语从句中还会出现than usual/than necessary这样的固定搭配，它们其实也是一种省略用法，比如：
They went to the office earlier than usual.
句子补充完整后相当于：
They went to the office earlier than (it was) usual (for them to go to the office).
当主句和从句中的谓语动词和时态一致，而主语不同时，经常会出现谓语部分省略的情况，比如：
He can run more quickly than his father (can).
Both recipes use more salt than mine (does).
在一些比较状语从句中主语和谓语也都可以省略，比如这个句子：
Wild animals spend less energy to live in winter than in summer.
这里than从句省略了主语和谓语，还原后相当于：
Wild animals spend less energy to live in winter than (they spend) in summer.
又比如《经济学人》文章里面有这样一个句子：
第二句补充完整后相当于：
End results were much better when managers were allowed to contribute than (end results were) when they were not (allowed to contribute).
需要注意的一点是，在than比较从句中有时候主语和谓语大部分都可以省略，但是介词短语、动词不定式、分词、动名词以及时间状语从句等结构都不省略，它们一般与than前面的部分形成平行结构，比如：
He has a higher opinion of Mary than of Jane.
这里的介词"of"不能省略，不然就破坏了than两边结构的平行。
又比如：
It is easier to understand a language than to speak it.
同时，如果than从句比较的是两个性质不同的事物，此时一般不使用省略，例如：
He speaks more quickly than his secretary can take dictation. （他说得很快，以至于秘书记不过来）
此时句子比较的两个对象是he speaks/ his secretary can take dictation，它们属于不同性质的事物，不能省略为：He speaks more quickly than his secretary.
除此之外，than从句中的省略有时候会造成歧义，我们在使用的时候要特别留意，比如：
Mr. Smith loves music more than his wife.
这句话也可以有两种理解：
(1) Mr. Smith loves music more than (he loves) his wife. （史密斯先生爱音乐胜过爱自己的妻子）
(2) Mr. Smith loves music more than his wife (does). （史密斯比他的妻子更喜欢音乐）
我们可以根据实际想表达的含义采取1句或2句的写法来消除歧义。
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