段落(paragraph)是由表达单一中心思想(controlling idea or central idea)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“certain rules”是指什么?展开句中通过四个“Don’t argue ——”逐一加以交代。从结构来看，这是一个比较典型段落，它包括了主题句，推展句和结论句(即本段中末句)。
1.1、主题句位置
主题句通常放在段落开端，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内容。主题句可以放在段中起到承上启下作用，或放在段尾起概括全段作用。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.
这段文字所讲主要是能源问题。第一句就概括性地提出“我们目前生活很大程度上依赖于能源”。而随后出现三句都是具体事例，对第一句进行说明和论述。我们可以断定第一句为该段主题句。
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.
1.2、 如何写好主题句中关键词
段落主题句对主题限定主要是通过句中关键词来表现。关键词要尽量写得具体些。对“具体”要求包括两个方面：一是要具体到能控制和限制段落发展;二是要具体到能说明段落发展方法。准确地把握关键词是清楚地表达段落主题、写好段落主题句重要前提之一。在上面例1，例2中，主题句关键词分别为：depends very much on energy;no one can avoid being influenced.
