句子插入题本来是属于托福阅读10种题型中比较简单的类型，但是许多同学都会因为缺乏对题型考点的认识以及对句子插入题一个本质和一个原则的忽略而导致失分很多。下面小编就来为大家具体讲解托福阅读句子插入题的本质和原则。
　　托福阅读句子插入题本质：插入句“可有可无”
　　本题的出题位置大家比较清楚，一般处于倒数第二题的位置，并且用来出题的段落是大家应该在前面的题目中已经见到过，而之前的题目和文章既然大家理解起来没有出现大的障碍，就说明插入进来的句子是一句可有可无的话。这就要从两个方面来理解了。
　　何为“可有”?
　　既然插入句能放进原文中去，就说明它和原文是有一定关系的，所以叫“可有”，这也就意味着，插入句的部分含义是在原文中有所重复的。所以我们的重点是回到文中找那部分被重复的句意。例如：
　　Watt's steam engine soon showed what it could do. It liberated industry fromdependence on running water. The engine eliminated water in the mines by drivingefficient pumps, which m-ade possible deeper and deeper mining. The ready availability ofcoal inspired William Murdoch during the 1790s to develop the first new form of nighttimeillumination to be discovered in a mill-ennium and a half. Coal gas rivaled smoky oil lamps andflickering candles, a-nd early in the new century, well-to-do Londoners grew accustomed togaslit houses and even streets. Iron manufacturers, which had starved for fuel while dependingon charcoal, also benefited from ever-increasing supplies of coal: b-last furnaces with steam-powered bellows turned out more iron and steel for the new machinery. Steam became themotive force of the Industrial Revolu-tion as coal and iron ore were the raw materials.
　　(托福真题6-Powering the Industrial Revolution)
　　Look at the four squares [] that indicate where the following sentence coul-d be added tothe passage.
　　The factories did not have to go to the streams when power could come to th-e factories.
　　Where would the sentence best fit?
　　在上题中，“liberated industry from dependence on running water”和 “The factories did not have to go to the streams”句意便发生了重合。所以我们可以确定，文中第二句话的前后应该是我们可以选择的范围。
　　何为“可无”?
　　这个便是经常被大家忽略的地方。既然这句话，可以省略，那么这个插入句究竟要符合什么特点呢?还是以上题为例，我们发现 “liberated industry from dependence on running water”和 “The factories didnot have to go to the streams”句意发生了重合，那究竟应该把插入句放在第二个还是第三个呢?这时就需要分情况讨论了：