托福考試的閱讀部分對考生英語閱讀能力的考察是全方位的，十大題型分別從詞匯、句子、段落和篇章這四個不同維度對考生是否清晰讀懂文章進行考察，而這其中長難句的理解是讓很多考生覺得頭痛的一個部分。想要攻克長難句，我們首先需要了解一些句子結構最基本的知識點。
一、英語句子結構的原則
1、謂動單一性原則 在一個句子里，有且只有一個謂語動詞。
2、主句單一性原則 在一個句子中，有且只有一個主句。(從句可以有若干個)
二、三大從句
1、名詞性從句 主語從句 賓語從句 同位語從句
引導詞 (what / how / that / why / whether)
結構 主語從句
what + V O
what + S V
形式賓語 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.(形式主語居多)
形式主語和強調句的區別
形式主語 It + v + (that +SVO)=n.
n.=it
強調句 It is / was + A + that + B
SVO=A+B
而且通常情況下 It is / was……是強調句
同位語從句
同位語的實質 n1,n2—n1=n2
S,n,VO.=S,引+svo,VO
前面的成分不應當在后面充當成分。
The fact, that the ancestors of birds are dinosaurs, is known.
可接同位語的名詞多為抽象名詞，例如：view / idea / suggestion / fact / reason / conclusion……
同位語和定語從句的區別
同位語的句子中，前面的成分不應當在后面充當成分。
The fact, that the sun is round.
定語從句中，引導詞充當成分。
The book, that you bought for me. “that”充當賓語。
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.
人們普遍接受，Pangaea 以一個特別大的陸地形式存在，后來他被分為兩個大塊，在南邊的Gondwanaland 和在北邊的Laurasia，他和那些在不同地殼上的大陸分開了并且向不同方向上漂移。
※如何找出復雜句中的謂語?
先找引導詞，然后去掉隨后的動詞，還有動詞的話，這個動詞就是謂語。
2、形容詞性從句=定語從句
引導詞
(1) that / which / who(whom)(代詞性)
(2) whose / when / where(非代詞性)
結構
that / which / who(whom)=S+VO=a.
This is pig that / which is very fat.
(1) that / which / who(whom)=O+SV=a.
因為代詞性的引導詞可以充當主語或賓語
This is the pig that/which I ate.(作賓語可省略引)
This is the pig from which I make fun.
引導詞前的介詞取決于后面的動詞
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(非代詞性)+SVO=a.
The book, whose cover is red, is quite interesting.
This is the place where(=in which 定語從句) I grew up. When 用在后面也可能是狀從,也有可能是定從。
※具體分析舉例
In his hypothesis that he developed based on it……
看上去該句的based 是一個n-ed的形式，但是她又是修飾誰呢?In his hypothesis() he developedthat based on it….
因此可以看出，based 修飾that，而在此句中，that指代 hypothesis.
※形容詞性從句的省略
當that / which在定語從句中充當賓語時，可將其省略。
This is the pig that/which I ate.
This is the pig I ate.
當that / which在定語從句中充當主語時，且從句的謂語動詞為be動詞時，可將其同時省略。
The house, which was built in 1919,was destroyed.
The house, built in 1919,was destroyed.
※個別情況下，which / as在引導定語從句時，也指代前面整個一句話。
As the plates drifted, they may have diverged, which(指代前面一句話) was associated withthe spread of the seafloor, or they may have converged, which(指代前面一句話) resulted incollision, subduction, and mountain building.
※系表倒裝
主系表結構 變成 表系主 結構成為系表倒裝只限于介詞詞組在句首時
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、副詞性從句 = 狀語從句
引導詞 when / though / while / although……
結構 when+s’+v’+adj/v-ing/v-ed,SVO.
When he was young, Jack was always beaten by his father.
省略的條件 s’=S v’=be
省略 When young, Jack was always beaten by his father.
省略 Other(聯系同一類的名詞，也就是說前面提到了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.
