一般而言，托福阅读中的说明文会说明某个现象，文章的首段是：介绍背景及引出下文;之后的主体段，结构也很清晰：主旨句Topic sentence + 解释句Explain + 举例子Examples + 总结句Summary。下面来看实例解析：

　　The Roman Army's Impact on Britain(选自官方真题Official-19)

　　In the wake of the Roman Empire's conquest of Britain in the first century A.D., a large number of troops stayed in the new province, and these troops had a considerable impact on Britain with their camps, fortifications, and participation in the local economy. Assessing the impact of the army on the civilian population starts from the realization that the soldiers were always unevenly distributed across the country. Areas rapidly incorporated into the empire were not long affected by the military. Where the army remained stationed, its presence was much more influential. The imposition of a military base involved the requisition of native lands for both the fort and the territory needed to feed and exercise the soldiers' animals. The imposition of military rule also robbed local leaders of opportunities to participate in local government, so social development was stunted and the seeds of disaffection sown. This then meant that the military had to remain to suppress rebellion and organize government.

　　these troops had a considerable impact on Britain，也就是首句说的是罗马军队对英国有影响。这句话相当于对题目的阐述，交代了文章的整个大背景。之后2～4句其实交代前提，开始具体的说开，要估算军队对平民的影响首先要了解一个事实：士兵不是平均分布的。那既然不是平均分布，肯定是有多有少的。所以，第三句接着说了有些地方士兵少：很快融入帝国的区域;第四句说有些地方士兵多影响大：军队驻扎的地方。第5～7句说回重点，也就是what，到底有哪些影响：军营先要征地;然后会抢走当地领导参与政府的机会;之后社会发展受阻，人民不满而反抗，最后军事镇压。

　　首段的首句话，虽然句子很长，但其实只需要抓住结构层次进行分析就能找到阅读重点。

　　所以总共7句话的段落，分层三个层次，主旨句+前提说明+重点阐述。事实上，段落首句就完成了background+introduction双重目的：英国被罗马帝国征服+ 士兵的营地/防御工事以及参与当地经济所造成的影响。

　　Economic exchange was clearly very important as the Roman army brought with it very substantial spending power. …… Such activities had a major effect on the local area, in particular with the construction of infrastructure such as roads, which improved access to remote areas.

　　第二段一开始就交代了经济交换的影响：强大的购买力，以及帮助修建基础设施。

　　Each soldier received his pay, but in regions without a developed economy there was initially little on which it could be spent. …… These settlements acted like small towns, becoming centers for the artisan and trading populations.

　　第三段还是在讲经济：刚来的时候士兵有钱没地方花;最后军队驻扎地变成了小镇。

　　The army also provided a mean of personal advancement for auxiliary soldiers recruited from the native peoples, as a man obtained hereditary Roman citizenship on retirement after service in an auxiliary regiment. …… By the later Roman period, frontier garrisons (groups of soldiers) were only rarely transferred, service in units became effectively hereditary, and forts were no longer populated or maintained at full strength.

　　第四段开始讲另一个重大影响：当地人通过当兵获得可继承的罗马公民地位。

　　This process of settling in as a community over several generations, combined with local recruitment, presumably accounts for the apparent stability of the British northern frontier in the later Roman period. It also explains why some of the forts continued in occupation long after Rome ceased to have any formal authority in Britain, at the beginning of the fifth century A.D. The circumstances that had allowed natives to become Romanized also led the self-sustaining military community of the frontier area to become effectively British..

　　第五段，做了个小总结，讲这种融合过程及当地招募促进了北部边境的稳定。

　　通过上文实例讲解，相信大家对于托福阅读说明文的基本结构和解题思路也有了一定了解。更多SAT干货敬请关注新航道SAT考试频道。