段落是文章的缩影。写好了段落，就能比较成功地写出较好的文章。经过中学英语基础阶段的学习之后，大学英语的写作重点就从选词造句转移到了联句成段和联段成篇上。进行段落层次上的写作训练，是写好英语文章的关键一环。下面介绍的是段落结构。
段落是由表达单一中心思想的一组句子构成，是文章结构的基本独立单位。本讲我们将就主题句和中心思想这两个方面展开讨论。
主题句与推展句
1． 主题句
主题句是表达段落主题的句子。它用以概括段落大意，要求全段其他文字都围绕它展开。请看下例：
My mother has passed along to me certain rules for getting along with others. Dont argue with parents; they will think you dont love them. Dont argue with children; they will think themselves victimized. Dont argue with spouses; they will think you are a tiresome mate. Dont argue with strangers; they will think you are not friendly. My mothers rules, in fact, can be summed up in two words: Dont argue.
主题句中提出的 certain rules 指的是什么？展开句中通过四个 Dont argue -- 逐一加以交代。从结构来看，这是一个比较典型的段落，它包括了主题句，推展句和结论句。
1．1 主题句的位置
主题句通常放在段落的开端，其特点是开门见山地摆出问题，然后加以详细说明。其作用是使文章的结构更清晰，更具说服力，便于读者迅速地把握主题和想象全段的内容。主题句可以放在段中起到承上启下的作用，或放在段尾起概括全段的作用。但初学者比较难于掌握，因而在四级考试中，考生应尽量采用将主题句放在段落开头的写作手法。
例1: Our life today depends very much on energy. In towns and in villages, on farms and in factories, machines have made life easier than it used to be. The machines use energy, and energy is needed for heating, lighting, communications, carrying goods -- everything. Factories and industrial plants use a great deal of energy to make the things that we use and buy and sell.
这段文字所讲的主要是能源问题。第一句就概括性地提出我们目前的生活很大程度上依赖于能源。而随后出现的三句都是具体事例，对第一句进行说明和论述。我们可以断定第一句为该段的主题句。
例2: No one can avoid being influenced by advertisements. Much as we may pride ourselves on our good taste, we are no longer free to choose the things we want, for advertising exerts a subtle influence on us. In their efforts to persuade us to buy this or that product, advertisers have made a close study of human nature and have classified all our little weakness.
以上这段由三句话组成。第一句是主题句，直截了当指出无人不受广告影响这个主题，接着列举两个推展句对其补充和支持，指出我们已无法自由选择所需的商品，理由是广告商已仔细研究了我们的心理，并完全掌握了我们的弱点。句子衔接自然，步步紧扣主题。
1．2 如何写好主题句中的关键词
段落的主题句对主题的限定主要是通过句中的关键词来表现的。关键词要尽量写得具体些。对具体的要求包括两个方面：一是要具体到能控制和限制段落的发展；二是要具体到能说明段落发展的方法。准确地把握关键词是清楚地表达段落主题、写好段落主题句的重要前提之一。在上面的例1，例2中，主题句的关键词分别为：depends very much on energy; no one can avoid being influenced. 我们再看下列例句：
原句1: He can fix a bicycle himself.
斜体部分应为主题句中的关键词。但它只是限制了段落发展的部分内容，并没有告诉读者该用哪种方法展开，是用因果关系法还是用分类法？
修正：He can fix a bicycle himself in several simple steps.
修正：There are several reasons why he can fix a bicycle himself.
原句2: She tries to improve her looks.
斜体部分应为主题句中的关键词。她试着改变她面容的理由是什么？或者她采取什么方法来改善呢？我们无法从关键词中清楚得知。
修正：She tries many ways to improve her looks.
修正：There are some reasons why she tries to improve her looks。
1.3 如何写好主题句的中心思想
主题句由两部分组成，即主题(topic)和中心思想。中心思想的作用是导向(control)和制约(limit)。我们前面谈到的关键词就反映了中心思想。所谓导向就是规定段落的发展脉络，所谓制约就是限制主题的覆盖范围，两者不可分割。没有导向，内容就会离题或偏题；没有制约，内容就可能超出一个段落所能容纳的范围。
对于初学写作的人来说，导向的重要性容易理解，但如果由他们自己来构思一个主题句，就可能忽视制约这一面。例如：
There are two ways in which one can own a book. The first is the property right you establish by paying for it, just as you have made it a part of yourself and the best way to make yourself a part of it is by writing in it.
本段的主题句如果没有in which 引出的定语从句，那么two ways 不仅不能起制约作用，而且也不能起导向作用，句子本身读起来也就使人觉得欠缺点什么。
Exercise is beneficial to your heart. A 22-year study was conducted by doctors in California. They found that people who work at physical jobs experience fewer heart attacks than other people. These active people work all the time at moderate speeds, their daily routine gives them an adequate amount of exercise and helps them stay in shape.
Exercise is beneficial这是毫无疑义的。但主题句中如不加上to your heart来加以 control 和 limit，那就流于空泛。
因此，紧紧把握主题句中controlling idea的导向和制约作用，是短文写作成功关键之一。
2．推展句
2.1 主要推展句
主要推展句(major supporting statement)的主要特点是：围绕段落主题句展开的每一个推展句本身都不要求作进一步的说明或证明，句与句之间的关系是相互独立又是互相连接的。
例1：(主题句) There are several factors affecting climate. (推展句1)One factor is the mount of sunlight received. (推展句2)Altitude, or the height above sea level, also determines climate. (推展句3)The oceans have an effect on the climate of adjacent land. (推展句4)In addition, climate is influenced by the general circulation of the atmosphere.
主题句指出影响气候的几个因素。然后用四个扩展句说明四种因素。第一种是太阳光的接收量，第二种是海拔高度，第三句和第四句分别是海洋和大气环流因素。
2.2 次要推展句
次要推展句(minor supporting statement)是指对主要推展句作进一步的事实分析和举例说明。它从属于某一个或某几个推展句。
例2：(主题句)I dont teach because teaching is easy for me. (主要推展句1)Teaching is the most difficult of the various ways I have attempted to earn my living: mechanic, carpenter, writer. (主要推展句2)For me, teaching is a red-eye, sweaty-palm, sinking-stomach profession. (次要推展句1)Red-eye, because I never feel ready to teach no matter how late I stay up preparing. (次要推展句2)Sweaty-palm, because Im always nervous before I enter the classroom, sure that I will be found out for the fool that I am. (次要推展句3)Sinking-stomach, because I leave the classroom an hour later convinced that I was even more boring than usual.
从属于主要推展句2的三个次要推展句起着解释说明作用，分别解释red-eye, sweaty-palm, sinking-stomach的含义，这就更加形象生动地证明了 teaching is not easy 这个主题。
2．3 主要推展句与次要推展句的关系
主要推展句与次要推展句的关系(relation)基本可以遵循下面的三部分(three-part)规则。
1〕每个主要推展句都应该是对主题句中表示主要思想的关键词的直接、明确的说明。
2〕每个次要推展句都应该说明它的主要推展句。
3〕含有讨论说明或分析的问题通常既要有主要推展素材，又要有次要推展素材。
2．4 写好推展句的方法
主题句及关键词确定后，开始选择和主题有关的信息和素材。实质上，针对关键词测试每一个所选择的素材就是一个分类的过程。有一种常用的方法就是句子展开前加以设问，然后解答，即设问－解答(why-because)的方法。下面我们通过举例来看一看这种分类过程是如何完成的。例3：
假设 (suppose) Topic sentence: English is an international language?
设问(why) Why can we say English is an international language?
解答(because)
Because: English is spoken by pilots and airport control operators on all the airways of the world.
Because: Over 70 percent of the worlds mail is written in English.
Because: More than 60 percent of the worlds radio programs are in English.
Because: pakistan was once one of the British colonies.
Because: Many members of the International Language Society are selected from English-spoken countries.
从上面可以看出，最初的三个推展句是和关键词an international language一致的。第四句和主题句不统一，尽管第四句会成为另一主题句(Many pakistanis speak English)很好的推展句。第五句说服力不强。
当然，在写成段落时，没有必要在每个句子开头写上 because，但是，在动笔展开句子时，头脑里要想着这个词，这种检验方法能帮助你把注意力集中在某个重心，围绕主题思想层层展开。
上面我们讨论了主要推展句的一种展开方法。而展开次要推展句的方法与主要推展句类似，这里我们就不再复述了。