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（ paragraph ）是由表达单一中心思想（controlling idea or central idea ）的一组句子（包括主题句 topic sentence,推展句supporting sentences及结论句concluding sentence）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。

英文写作中需要注意的主题句与推展句

1． 主题句
　　主题句（topic sentence）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. Don't argue with parents; they will think you don't love them. Don't argue with children; they will think themselves victimized. Don't argue with spouses; they will think you are a tiresome mate. Don't argue with strangers; they will think you are not friendly. My mother's rules, in fact, can be summed up in two words: Don't argue.
　　主题句中提出的 "certain rules" 指的是什么？展开句中通过四个 "Don't 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（即本段中的末句）。　　
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（斜体部分）。

　　例2: （主题句）No one can avoid being influenced by advertisements. （推展句1）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. （推展句2）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。