句子插入题是托福阅读中的一道必考题型，题量不多，每篇文章中出现一道。句子插入题是给出一句话让我们来判断这句话应该加入1-2个段落中的四个黑方块的哪个部分，用鼠标点击所选的黑色方块即可。做这类题目的关键首先是理解待插入的句子，寻找其中的衔接词，分析这个句子的特点和其在文中的作用，然后把这个句子带入原文中检验其是否合适。下面朗阁海外考试研究中心的专家将就句子插入题的解题步骤做出详细的说明。
 
一、解题步骤与解题要点
 
Ø  阅读并分析待插入的句子，找出其中的线索词；
Ø  从*个小黑方块前一句开始读，逐一代入进行验证；
Ø  插入句必须满足前后两条线索，先前再后。（先用前线索再看后线索）
从此步骤中可以看出掌握线索词是解答此题的关键，那么线索词都有哪些？主要有两大类，指代线索和逻辑线索。
 
1. 指代线索
(1). 指代词(they, it, them)必然有所指代；指代词(this, these, their, other, another, such)后面若有名词短语，则向前寻找此名词短语或其同义改写。
 
例题1:
Paragraph 4: Regularly flowing rivers and streams that originate within arid lands are known as "endogenous." These are generally fed by groundwater springs, and many issue from limestone massifs, such as the Atlas Mountains in Morocco. Basaltic rocks also support springs, notably at the Jabal Al-Arab on the Jordan-Syria border. ■Endogenous rivers often do not reach the sea but drain into inland basins, where the water evaporates or is lost in the ground. ■Most desert streambeds are normally dry, but they occasionally receive large flows of water and sediment. ■
Paragraph 5: Deserts contain large amounts of groundwater when compared to the amounts they hold in surface stores such as lakes and rivers. ■But only a small fraction of groundwater enters the hydrological cycle—feeding the flows of streams, maintaining lake levels, and being recharged (or refilled) through surface flows and rainwater. (TPO 16 Water in the Desert)
 
12. Look at the four squares [■] that indicate where the following sentence could be added to the passage.
These sudden floods provide important water supplies but can also be highly destructive.
Where would the sentence best fit?
 
解析：
1. 分析黑体句，找到其中的线索词即为these sudden floods, 则要插入的方块前面必定要出现名词短语sudden floods或是其同义改写。
2. 从*个小黑方块前一句开始读，寻找与线索词相吻合的信息。
3. 在第三个方块前面出现了large flows of water and sediment（巨大的水流和沉积物），即为sudden floods的同义替换，所以答案应当是第三个黑方块的位置。
 
例题2:
Paragraph 4: One of the outer planets, Saturn, has a density of only 0.7 that of water, which means that Saturn would float in water. Variations in the composition of the planets are largely responsible for the density differences. ■The substances that make up both groups of planets are divided into three groups—gases, rocks, and ices—based on their melting points. ■The terrestrial planets are mostly rocks: dense rocky and metallic material, with minor amounts of gases. ■The Jovian planets, on the other hand, contain a large percentage of the gases hydrogen and helium, with varying amounts of ices: mostly water, ammonia, and methane ices. ■ (TPO 16 Planets in Our Solar System)
 
12. Look at the four squares [■] that indicate where the following sentence could be added to the passage.
This explains their relatively low densities.
Where would the sentence best fit?
 
解析：首先分析待插入的句子，涉及两个指代词，this和their, 根据这两个指代词则能够推出前文一定要讲到某些密度较低的行星（即their所指代的），并且前面还要给出密度低的原因（即this所指代的）。然后阅读段落，发现段落中首先提到行星构成成分的不同会导致密度的差异，然后讲到类地行星(terrestrial planets)主要由密度大的石头组成，然后提到类木行星(Jovian planets)是由大量的气体组成的，所以正确答案应是第四个黑方块的位置。
 
(2). 定冠词the+名词，用于重复上文提到的事物
 
例题1:
Paragraph 3: Film analysis of the infant's responses, heart and respiration rate monitors, and nonnutritive sucking devices are used as effective tools in understanding infant perception. ■Film analysis permits researchers to carefully study the infant's responses over and over and in slow motion. ■Precise measurements can be made of the length and frequency of the infant's attention between two stimuli. ■Heart and respiration monitors provide the investigator with the number of heartbeats or breaths taken when a new stimulus is presented. ■Numerical increases are used as quantifiable indicators of heightened interest in the new stimulus. Increases in nonnutritive sucking were first used as an assessment measure by researchers in 1969.
 
13. Look at the four squares [■] that indicate where the following sentence could be added to the passage.
The repetition allows researchers to observe the infant's behavior until they reach agreement about the presence and the degree of the infant's response.
Where would the sentence best fit?
 
解析：分析黑体句，找到线索词the repetition, 根据规则前文中应出现repetition这个信息，从*个小黑方块前一句开始分析，发现第二个黑方块前面出现了…over and over and in slow motion, 识别出是repetition的同义改写，则答案即为第二个小黑方块位置。
 
例题2:
The universal global warming at the end of the Ice Age had dramatic effects on temperate regions of Asia, Europe, and North America. Ice sheets retreated and sea levels rose. ■The climatic changes in southwestern Asia were more subtle, in that they involved shifts in mountain snow lines, rainfall patterns, and vegetation cover. ■However, these same cycles of change had momentous impacts on the sparse human populations of the region. ■At the end of the Ice Age, no more than a few thousand foragers lived along the eastern Mediterranean coast, in the Jordan and Euphrates valleys. Within 2,000 years, the human population of the region numbered in the tens of thousands, all as a result of village life and farming. ■Thanks to new environmental and archaeological discoveries, we now know something about this remarkable change in local life.
 
13. Look at the four squares [■] that indicate where the following sentence can be added to the passage.
One of the major effects was the rapid growth of the human population itself.
Where would the sentence best fit?
Click on a square [■] to insert the sentence in the passage.
 
解析：分析待插入的句子，可以找到线索词the major effects, 则推出前文中一定要提到effects或其同义替换，本题中还有一个后线索，后面的句子中应该要提到人口的迅速增长这个信息。阅读段落，发现these same cycles of change had momentous impacts on the sparse human populations of the region中impacts即为effects的同义替换，并且第三个黑方块后面的两句话都在详细论述人口从*初的几千人增加到上万人，所以前后都符合线索词，则正确答案为第三个黑方块。
 
2. 逻辑线索
在逻辑线索中主要会出现以下几种情况：
(1). 插入句中出现表结果的表达：一般可以排除段首位置，注意因果的逻辑关系
表示因果关系的信号词：therefore, thus, accordingly, consequently, hence, so, as a result等。
 
例题：
Paragraph 3: Another explanation for the focus on animals might be that these people sought to improve their luck at hunti