段落(paragraph)是由表達單一中心思想(controlling idea or central idea)一組句子(包括主題句 topic sentence，推展句supporting sentences及結論句concluding sentence)構成，是文章結構基本獨立單位。本講我們將就主題句和中心思想這兩個方面展開討論。
主題句與推展句
1、主題句
主題句(topic sentence)是表達段落主題句子。它用以概括段落大意，要求全段其他文字都圍繞它展開。請看下例：
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.
主題句中提出“certain rules”是指什麼?展開句中通過四個“Don’t argue ——”逐一加以交代。從結構來看，這是一個比較典型段落，它包括了主題句，推展句和結論句(即本段中末句)。
1.1、主題句位置
主題句通常放在段落開端，其特點是開門見山地擺出問題，然後加以詳細說明。其作用是使文章結構更清晰，更具說服力，便於讀者迅速地把握主題和想象全段內容。主題句可以放在段中起到承上啓下作用，或放在段尾起概括全段作用。但初學者比較難於掌握，因而在四級考試中，考生應儘量採用將主題句放在段落開頭寫作手法。
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.
這段文字所講主要是能源問題。第一句就概括性地提出“我們目前生活很大程度上依賴於能源”。而隨後出現三句都是具體事例，對第一句進行說明和論述。我們可以斷定第一句爲該段主題句。
例2：(主題句)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、 如何寫好主題句中關鍵詞
段落主題句對主題限定主要是通過句中關鍵詞來表現。關鍵詞要儘量寫得具體些。對“具體”要求包括兩個方面：一是要具體到能控制和限制段落髮展;二是要具體到能說明段落髮展方法。準確地把握關鍵詞是清楚地表達段落主題、寫好段落主題句重要前提之一。在上面例1，例2中，主題句關鍵詞分別爲：depends very much on energy;no one can avoid being influenced.
