今天我们学习托福阅读中另一种常见的段落逻辑框架-转折结构，用于否定或修正前文的观点，同时引出新的观点或事实，常见于涉及科学理论或历史事件的段落。这种结构通常通过转折词（如However, But, Nevertheless等）来实现，重点在于后文的观点往往更重要或更可信，可以简要记忆为“虽然但是看但是”。
如何识别出转折结构呢？我们可以用到以下方法：
托福阅读识别显性逻辑词
强转折：
however, nevertheless, nonetheless, yet, but
让步转折：
although, even though, while, despite
对比标志：
in contrast, on the other hand, conversely
托福阅读语义转折强度
强转折：
新观点推翻前文结论（常用however/but开头）
弱转折：
补充限制条件或特例（常嵌套在段落中间或结尾）
句式：
通常先陈述一个观点，然后通过转折词引出另一个观点以反驳或修正前文观点。
托福阅读例题解析
The undisputed pre-Columbian presence in Oceania of the sweet potato, which is a New World domesticate, has sometimes been used to support Heyerdahl’s “American Indians in the Pacific” theories. However, this is one plant out of a long list of Southeast Asian domesticates.
As Patrick Kirch, an American anthropologist, points out, rather than being brought by rafting South Americans, sweet potatoes might just have easily been brought back by returning Polynesian navigators who could have reached the west coast of South America.
这一段没有上下文乍看难以理解，pre-Columbian, Oceania, New World, American Indians in the Pacific涉及的地理、历史概念阻碍了内容含义的抓取，但是通过However这一逻辑词，可以发现前文是用红薯的存在证明了H（Heyerdahl）的理论，而后文则是对这一说法的驳斥。所以，通过段落结构的分析，可以得到段落支持Kirch的观点，即红薯的传播可能与波利尼西亚航海者有关，而非美洲印第安人。
It would appear that the instability of the climatic conditions led populations that had originally been nomadic to settle down and develop a sedentary style of life, which led in turn to population growth and to the need to increase the amount of food available.
Farming originated in these conditions. Later on, it became very difficult to change because of the significant expansion of these populations. It could be argued, however, that these conditions are not sufficient to explain the origins of agriculture. "Earth had experienced previous periods of climatic change, and yet agriculture had not been developed. "
先提出一个看似合理的观点（“It would appear that...”），气候变化的不稳定性导致原本游牧的群体定居下来，发展出定居生活方式，进而导致人口增长和对食物的需求增加，最终促成了农业的起源；然后通过转折词（“It could be argued, however...”）引出相反的观点或反驳，指出这些条件不足以解释农业的起源，因为地球以前也经历过气候变化，但农业并未出现，因此农业的起源可能还有其他更复杂的因素，而不仅仅是气候变化。
与此相似的结构还有：
It would/might appear that... However...
It looks/seems like... But...
At first glance... However...
One might think... But...
The initial hypothesis... Later findings...
过去观点与现在思想的转折
Previously/Once/Earlier interpretations...Now/Recent studies, however...
The commercialization of lumbering was in part the product of technological change【主旨句】. The early, thick saw blades tended to waste a large quantity of wood, with perhaps as much as a third of the log left behind on the floor as sawdust or scrap.
In the 1870s, however, the British-invented band saw, with its thinner blade, became standard issue in the Great Lakes states' lumber factories. Meanwhile, the rise of steam-powered mills streamlined production by allowing for the more efficient, centralized, and continuous cutting of lumber.
Steam helped to automate a variety of tasks, from cutting to the carrying away of waste. Mills also employed steam to heat log ponds, preventing them from freezing and making possible year-round lumber production.
主旨句：木材商业化的部分原因是技术变革。
第一层：描述早期木材加工过程中存在的浪费问题：厚锯片浪费了大量木材。
第二层：从问题转向解决方案，用薄锯片这样的技术变革解决问题，之后再进一步介绍蒸汽动力锯木厂的兴起及其带来的好处。
警惕一个机械操作："看到but就分段"。然而，转折词可能仅服务于局部细节对比（句内逻辑），而非整体观点转向（段落结构）。例如：
The traditional view holds that dinosaurs were cold-blooded. However, recent discoveries of high-growth-rate bone structures but limited metabolic evidence complicate this classification.
However标志观点转向（传统观点→新发现）-转折结构
but仅连接并列证据（高生长率骨骼 vs 有限代谢证据）-句内逻辑
同学们容易将转折结构和对比结构混淆，两者都涉及两种观点或事物的比较，但它们的侧重点和逻辑关系有所不同。
转折结构是先提出一个观点，然后反驳这个观点；用于反驳、修正或提供替代解释。
先说“苹果可以有助于健康”，然后话锋一转说“但是过量食用苹果会导致某些疾病”，是转折结构；
对比结构则是同时呈现两种不同的观点或事物，突出两者的差异；用于展示不同的情况或角度，不一定要否定其中一方。
呈现苹果和橘子在口感、用途、功效的差异则是比较结构。
下次我们会就对比结构展开更进一步的解释~
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