1 比较结构用法
On the other hand, if the bird carried the shellfish up to a higher altitude, an altitude higher than it is necessary to crack the shell, it'd be wasting energy.
老师您好，请问an altitude higher than it is necessary to crack the shell如何理解？我不明白的地方是：这个it是形式主语吗？即it is necessary to crack the shell=to crack the shell is necessary.翻译：高度高于打破贝壳是有必要的。这明显不是一个通顺的短语。是省略某些内容吗？
第二个问题是：我明白than 可以作为准关系代词， 例如 I have more money than is needed= I have more money than the money which is needed. 类似地，an altitude higher than it is necessary to crack the shell 是否可以改写为：an altitude higher than an altitude which is necessary to crack the shell ？
最佳答案 2019-06-18 01:15
an altitude higher than it is necessary to crack the shell,
it 为从句的形式主语，真正主语为不定式。than隐含了the attitude at which, 作不定式的状语。than分句意为：摔碎贝壳所需的高度，名词短语意为，比摔碎贝壳所需高度更高的一个高度。
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其它 1 个回答
than 后面，大致有
X can provide better functions than _______
what is possible with Y.
the function that is possible with Y
(what) it is possible to provide with Y
the function that it is possible to provide with Y.
（里面还有一些省略的情况，这里略去。有兴趣的可以参考CGEL P1111）
On the other hand, if the bird carried the shellfish up to a higher altitude, an altitude higher than it is necessary to crack the shell, it'd be wasting energy.
黑体部分，是虚拟语气，说明鸟很聪明，它能够掌握必要的高度，免得多费劲
On the other hand, if the bird carried the shellfish up to a higher altitude, an altitude higher than it is necessary to crack the shell, it'd be wasting energy.
粗体部分的理解上，可在粗体部分前插入一个词“for example”
On the other hand, if the bird carried the shellfish up to a higher altitude, an altitude higher than it is necessary to crack the shell, it'd be wasting energy.
一般有to do sth的，最好用it is...这种形式配搭，比较清楚。
一般把这种than 看成 than (what) 都不失为简明的办法：
Parents had better not give kids more money than is needed.
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