托福考试的阅读部分对考生英语阅读能力的考察是全方位的，十大题型分别从词汇、句子、段落和篇章这四个不同维度对考生是否清晰读懂文章进行考察，而这其中长难句的理解是让很多考生觉得头痛的一个部分。想要攻克长难句，我们首先需要了解一些句子结构最基本的知识点。

　　一、英语句子结构的原则

　　1、谓动单一性原则 在一个句子里，有且只有一个谓语动词。

　　2、主句单一性原则 在一个句子中，有且只有一个主句。(从句可以有若干个)

　　二、三大从句

　　1、名词性从句 主语从句 宾语从句 同位语从句

　　引导词 (what / how / that / why / whether)

　　结构 主语从句

　　what + V O

　　what + S V

　　形式宾语 Make it possible for sb.to do

　　that / how / why / whether+S V O=n.

　　That the ancestors of birds are dinosaurs is known. It is known that the ancestors of birdsare dinosaurs.(形式主语居多)

　　形式主语和强调句的区别

　　形式主语 It + v + (that +SVO)=n.

　　n.=it

　　强调句 It is / was + A + that + B

　　SVO=A+B

　　而且通常情况下 It is / was……是强调句

　　同位语从句

　　同位语的实质 n1,n2—n1=n2

　　S,n,VO.=S,引+svo,VO

　　前面的成分不应当在后面充当成分。

　　The fact, that the ancestors of birds are dinosaurs, is known.

　　可接同位语的名词多为抽象名词，例如：view / idea / suggestion / fact / reason / conclusion……

　　同位语和定语从句的区别

　　同位语的句子中，前面的成分不应当在后面充当成分。

　　The fact, that the sun is round.

　　定语从句中，引导词充当成分。

　　The book, that you bought for me. “that”充当宾语。

　　Example

　　It is generally accepted that the single super continent known as Pangaea indeed existed, that Pangaea subsequently broke apart into two giant pieces, Gondwanaland in the south and Laurasia in the north, and that the continents attached to the various crustal plates separated and drifted in various directions.

　　人们普遍接受，Pangaea 以一个特别大的陆地形式存在，后来他被分为两个大块，在南边的Gondwanaland 和在北边的Laurasia，他和那些在不同地壳上的大陆分开了并且向不同方向上漂移。

　　※如何找出复杂句中的谓语?

　　先找引导词，然后去掉随后的动词，还有动词的话，这个动词就是谓语。

　　2、形容词性从句=定语从句

　　引导词

　　(1) that / which / who(whom)(代词性)

　　(2) whose / when / where(非代词性)

　　结构

　　that / which / who(whom)=S+VO=a.

　　This is pig that / which is very fat.

　　(1) that / which / who(whom)=O+SV=a.

　　因为代词性的引导词可以充当主语或宾语

　　This is the pig that/which I ate.(作宾语可省略引)

　　This is the pig from which I make fun.

　　引导词前的介词取决于后面的动词

　　this is the pig,which is very fast

　　this is the pig, (which)I ate.

　　This is the pig, from which I make fun.

　　(2) whose / when / where(非代词性)+SVO=a.

　　The book, whose cover is red, is quite interesting.

　　This is the place where(=in which 定语从句) I grew up. When 用在后面也可能是状从,也有可能是定从。

　　※具体分析举例

　　In his hypothesis that he developed based on it……

　　看上去该句的based 是一个n-ed的形式，但是她又是修饰谁呢?In his hypothesis() he developedthat based on it….

　　因此可以看出，based 修饰that，而在此句中，that指代 hypothesis.

　　※形容词性从句的省略

　　当that / which在定语从句中充当宾语时，可将其省略。

　　This is the pig that/which I ate.

　　This is the pig I ate.

　　当that / which在定语从句中充当主语时，且从句的谓语动词为be动词时，可将其同时省略。

　　The house, which was built in 1919,was destroyed.

　　The house, built in 1919,was destroyed.

　　※个别情况下，which / as在引导定语从句时，也指代前面整个一句话。

　　As the plates drifted, they may have diverged, which(指代前面一句话) was associated withthe spread of the seafloor, or they may have converged, which(指代前面一句话) resulted incollision, subduction, and mountain building.

　　※系表倒装

　　主系表结构 变成 表系主 结构成为系表倒装只限于介词词组在句首时

　　1、My hometown lies in Jilin province.

　　In Jilin province lies my hometown.

　　2、A,B,C….are among the species of seabirds.

　　Among the species of seabird are A,B,C….

　　3、副词性从句 = 状语从句

　　引导词 when / though / while / although……

　　结构 when+s’+v’+adj/v-ing/v-ed,SVO.

　　When he was young, Jack was always beaten by his father.

　　省略的条件 s’=S v’=be

　　省略 When young, Jack was always beaten by his father.

　　省略 Other(联系同一类的名词，也就是说前面提到了chemical defenses) possible chemical defenses, while (they are 省略) not directly toxic to the parasite, may inhibit some essential step in the establishment of a parasitic relationship. For example, glycol proteins in plant cell walls may inactivate enzymes the degrade cell walls.