很多考生在准备托福考试的时候，总会发现部分托福考试中的句子远远超出了自己的能力范围。这就是我们所说的托福阅读长难句，托福长难句在考试中最直接的运用就是考察插入句子题和解释句子题，下面，天道留学就为大家介绍攻克托福阅读长难句的四种逻辑关系。
　　1.因果关系
　　因：because, because of, for, as, since, in that, on account of, with
　　果：so, so that, therefore, thereby, as a result, hence, thus, consequently, accordingly
　　因果关系除了传统意义上的显性因果表达词外，隐性的因果同样是不可忽略的一个重要部分。
　　隐性因果：
　　A 导致(因-果)：cause, reason, lead to, give rise to, result in, render, make, let, ask, support, push, stimulate, spark, spur, fuel, produce, be responsible for
　　如：The increased pressures of expanding populations have led to the removal of woody plants so that many cities and towns are surrounded by large areas completely lacking in trees and shrubs.
　　在这段话中，有lead to，表示了导致的意思，即结果;而so that 更进一步表示了后面的结果，所以可以充分判定这段话有因果关系的逻辑。
　　B 由…而来(果-因)：result from, derive from, originate from, initiate from, stem from, be responsive to, be attributable to
　　如：“The extreme seriousness of desertification results from the vast areas of land and tremendous numbers of people affected, as well as from the great difficulty of reversing or even slowing the process.”
　　在这段话中，根据result from可以推断出有因果关系，那如果是解释句子题时，选项中有因果关系就可以优先考虑。
　　C 反映，体现(果-因)：reflect, present, demonstrate, suggest, imply, show This result demonstrates that…
　　D 考虑到：given, considering, in view of, thanks to, according to He succeeded thanks to (in view of) his effort.
　　E 依赖于：rely on, depend on, resort to, He resorted to books when he had problems.
　　F 条件关系：when, once, as soon as, as long as As soon as he got the money, he would leave the country at once.
　　G 分词短语，不定式做状语：Failing in the final exam, she cried.
　　2.对比转折关系
　　A 对比：while, whereas, on the other hand
　　在解释句子题、插入句子题中，一旦出现对比关系，学生在掌握的基础上就能非常快速的判定句间和句内的关系。While, whereas 前后连接的是平行结构，on the other hand前必定有on one hand，可以用来把握句间关系。
　　B 转折：but, although, despite, in spite of, nevertheless, however
　　转折是托福阅读的一个经典考点之一，掌握这些转折词对考试非常有帮助。
　　3.比较关系
　　A 同级比较 as…as
　　B 比较级：more…than, -er than, less…than
　　C 变化：change, alter, vary, modify, revise, increase, decrease, enhance, diminish, develop, progress, advance, improve, retreat, degenerate, continue, remain
　　D 差异：different, distinguish, separate, same, similar, comparable, compare…to E 超越：surpass, exceed, excel, over F 最高级 1)本身有最高级含义：maximum, minimum, peak, outstanding, top 2) 本身程度比较深：amazing, surprising, astonishing, prohibitively high 3) 否定+比较=最高级 No one is more outstanding than him.
　　从这句话中可以看出，否定加比较表示的是一种最高级关系。
　　4.否定关系
　　显性否定：no, not, never, nor, none neither 隐形否定：fail to, refuse, remove, miss, reject, absence of, lack of 否定前缀：a-, ab-, dis-, il-, im-, in-, non- , un- 否定前缀是词汇题中经常出的一个考点，把握否定前缀可以帮助考生把握一些生词，依靠否定前缀对选项进行一个排除。
　　双重否定：not fail to, not illegal, not uncommon, not unavailable 双重否定是英文中经常运用的表达方式，由于在平时中文对话中用的很少，随意对双重否定的把握就显得特别的重要。