1.定语(包含后置定语与定语从句)
2.同位语
3.并列结构
4.that引导的各种从句
5.插入结构
6.独立主格
7.倒装句
8.强调句
9.虚拟语气
对于不同类型的难句，把握主干的方法也不一样。以倒装句为例，主要有下列情况：
1.方位副词放在句首
Herein lay the beginning of what ultimately turned from ignorance to denial of the value ofnutritional therapies in medicine。
2.介词放在句首
Among the species of seabirds that use the windswept cliffs of the Atlantic coast of Canada in thesummer to mate, lay eggs, and rear their young are common murres, Atlantic puffins, black-leggedkittiwakes, and northern gannets。
3. 形容词放在句首
Implicit in it is an aesthetic principle as well: that the medium has certain qualities of beauty andexpressiveness with which sculptors must bring own aesthetic sensibilities into harmony。
4.过去分词放在句首
Accustomed though we are to speaking of the films made before 1927 as “silent”， the film hasnever been, in the full sense of the word, silent。
5.现在分词放在句首
Missing until recently were fossils clearly intermediate, or transitional, between land mammals andcetaceans。
句子倒装的目的主要有两个：
一是被提前的部分被强调，如上述的第四句：Accustomed though we are to speaking of the filmsmade before 1927 as “silent”，其中的accustomed按照正常语序应该放在are之后：Though we areaccustomed to speaking of the films made before 1927 as “silent”。之所以将accustomed提前，是为了强调它。
二是为了句子的平衡，否则将导致头重脚轻的效果，如上述的第一句、第三句和第五句。以第五句为例，按照正常语序应该是：Fossils clearly intermediate, or transitional, between land mammalsand cetaceans were missing until recently。但是这种句子结构比较糟糕，因为主语及其修饰语过长(从句首直到cetaceans)，而谓语部分were missing until recently较短，容易造成头重脚轻的效果。第五句将missing提前，避免了这一现象。
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