在托福阅读文章的段落内部，句子和句子并非出于同一个层次，因此需要运用句间关系来辨析段落中句子的地位。对于单独的一个段落，不考察在文章中的地位(或与其他段落的段落关系)时，段落内部主要有Main Idea及Detail。下面前程百利小编将给大家详解介绍如何根据显性Main Idea和隐性Main Idea来分析托福阅读文章的段落结构。
1. 显性Main Idea及Topic Sentence的位置
Main Idea显性时，则会明确出现一句能够概括段落主要思想的句子，即Topic Sentence。如例8：
例8：1There are adaptations that increase the amount of forward thrust as well as those that reduce drag. 2Again, these fishes are the envy of engineers. 3Their high, narrow tails with swept-back tips are almost perfectly adapted to provide propulsion with the least possible effort. 4Perhaps most important of all to these and other fast swimmers is their ability to sense and make use of swirls and eddies (circular currents) in the water. 5They can glide past eddies that would slow them down and then gain extra thrust by "pushing off" the eddies. 6Scientists and engineers are beginning to study this ability of fishes in the hope of designing more efficient propulsion systems for ships.
解析：
句1：许多adaptations增加向前推进力的量
句2：工程师嫉妒这些鱼
句3：尾巴几乎是完美的适于提供推进力，用最少可能的功。
句4：感知并运用水流的能力最重要
句5：他们滑过水流并从中获得额外推进力
句6：科学家和工程师研究感知运用水流能力
根据句间关系，可以判断出：
句1为本段最概括的句子。因此为显性MI，即TS。
句2为引用工程师态度(与段落主要内容无直接关系)，侧面反映adaptations优越，MaD1。
句3与句1为概述与详述关系，为第一个MaD2，意思方面为尾巴。
句4与句3为并列关系，因此处于同一级别，与句1为概述与详述关系，为第二个MaD3。
句5与句4为抽象与具体关系，因此为第一个MiD1。
句6与句5为并列关系，与句4为抽象与具体关系(科学家与工程师都在研究这一能力，反映出该能力的important含义)，因此为第二个MiD2。
因此段落共分3个层次：
第1层次：TS(句1);
第2层次：MaD1(句2), MaD2(句3), MaD3(句4);
第3层次：MiD1(句5), MiD2(句6)。
值得注意的是：TS大多数出现在段首，如例8。但并非一定在段首，常在段中及段末出现。
下面举出两个在段中和段末的情况。
TS在段中
这种情况对于考生来说最难识别。在段中时，作者先给出介绍性内容，后给出TS。如例9。
例9：1Psychological researchers generally recognize that facial expressions reflect emotional states. 2In fact, various emotional states give rise to certain patterns of electrical activity in the facial muscles and in the brain. 3The facial-feedback hypothesis argues, however, that the causal relationship between emotions and facial expressions can also work in the opposite direction. 4According to this hypothesis, signals from the facial muscles ("feedback) are sent back to emotion centers of the brain, and so a person's facial expression can influence that person's emotional state. 5Consider Darwin's words: "The free expression by outward signs of an emotion intensifies it. On the other hand, the repression, as far as possible, of all outward signs softens our emotions." 6Can smiling give rise to feelings of good will, for example, and frowning to anger?
解析：
句1：心理研究一般认同面部表情反应情感状态。
句2：各种情感状态引起面部肌肉和大脑中的某些电活动的形式。
句3：然而，面部回馈理论认为情感和面部表情的因果关系也可以以反方向进行。
句4：根据这一理论，来自面部肌肉的信号反馈回大脑情感中心，并且因此一个人的面部表情能影响一个人的情感状态。
句5：达尔文说：情感的向外信号的自由表达加剧了情感;向外信号尽可能的压抑柔化了情感。
句6：微笑能引起善意的感受吗?皱眉会引起生气吗?
根据意思方面，可以判断出句间关系：
句1与句2意思方面相同，为抽象与具体关系，句2为句1的“反映”提供解释。
句3到句6意思方面相同，其中句3最具概述性。
因此本段句3为TS，句1和句2为介绍性内容。
TS在段末。
段末TS一般是对细节的概括性句子，或对某问题的总结，如例10。
例10：1Another major discovery was made in Egypt in 1989. 2Several skeletons of another early whale, Basilosaurus, were found in sediments left by the Tethys Sea and now exposed in the Sahara desert. 3This whale lived around 40 million years ago, 12 million years after Pakicetus. 4Many incomplete skeletons were found but they included, for the first time in an archaeocyte, a complete hind leg that features a foot with three tiny toes. 5Such legs would have been far too small to have supported the 50-foot-long Basilosaurus on land. 6Basilosaurus was undoubtedly a fully marine whale with possibly nonfunctional, or vestigial, hind legs.
解析：
句6相对于其他句子最具有概述性，因此句6为TS。
2. 隐性Main Idea
当Main Idea为隐性时，句中未出现明确的概述性句子(TS)。如例11。
例11：1Exhibitors, however, wanted to maximize their profits, which they could do more readily by projecting a handful of films to hundreds of customers at a time (rather than one at a time) and by charging 25 to 50 cents admission. 2About a year after the opening of the first Kinetoscope parlor in 1894, showmen such as Louis and Auguste Lumiere, Thomas Armat and Charles Francis Jenkins, and Orville and Woodville Latham (with the assistance of Edison's former assistant, William Dickson) perfected projection devices. 3These early projection devices were used in vaudeville theaters, legitimate theaters, local town halls, makeshift storefront theaters, fairgrounds, and amusement parks to show films to a mass audience.
解析：
句1：放映商想最大化利润。
句2：showmen完善projection devices。
句3：projection devices被使用在若干地点。
句1与句2为并列关系。
句2与句3为并列关系。
本段未出现概括段落的句子，须考生自己总结。即：projection devices出现的过程。
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