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。
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）。