主題句(topic sentence)是表達段落主題的句子。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.
主題句中提出的 "certain rules" 指的是什么?展開句中通過四個 "Don't argue --" 逐一加以交代。從結構來看，這是一個比較典型的段落，它包括了主題句，推展句和結論句(即本段中的末句)。
1.1 主題句的位置
主題句通常放在段落的開端，其特點是開門見山地擺出問題，然后加以詳細說明。其作用是使文章的結構更清晰，更具說服力，便于讀者迅速地把握主題和想象全段的內容。主題句可以放在段中起到承上啟下的作用，或放在段尾起概括全段的作用。但初學者比較難于掌握，因而在四級考試中，考生應盡量采用將主題句放在段落開頭的寫作手法。，
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.，
這段文字所講的主要是能源問題。第一句就概括性地提出“我們目前的生活很大程度上依賴于能源”。而隨后出現的三句都是具體事例，對第一句進行說明和論述。我們可以斷定第一句為該段的主題句(斜體部分)。 ，
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.
以上這段由三句話組成。第一句是主題句，直截了當指出無人不受廣告影響這個主題，接著列舉兩個推展句對其補充和支持，指出我們已無法自由選擇所需的商品，理由是廣告商已仔細研究了我們的心理，并完全掌握了我們的弱點。句子銜接自然，步步緊扣主題。
1.2 如何寫好主題句中的關鍵詞
段落的主題句對主題的限定主要是通過句中的關鍵詞來表現的。關鍵詞要盡量寫得具體些。對“具體”的要求包括兩個方面：一是要具體到能控制和限制段落的發展;二是要具體到能說明段落發展的方法。準確地把握關鍵詞是清楚地表達段落主題、寫好段落主題句的重要前提之一。在上面的例1，例2中，主題句的關鍵詞分別為：depends very much on energy; no one can avoid being influenced. 我們再看下列例句：
原句1: He can fix a bicycle himself.
斜體部分應為主題句中的關鍵詞。但它只是限制了段落發展的部分內容，并沒有告訴讀者該用哪種方法展開，是用因果關系法還是用分類法?
修正：He can fix a bicycle himself in several simple steps. 的
修正：There are several reasons why he can fix a bicycle himself.
原句2: She tries to improve her looks.
斜體部分應為主題句中的關鍵詞。她試著改變她面容的理由是什么?或者她采取什么方法來改善呢?我們無法從關鍵詞中清楚得知。
修正：She tries many ways to improve her looks. 的
修正：There are some reasons why she tries to improve her looks。
1.3 如何寫好主題句的中心思想
主題句由兩部分組成，即主題(topic)和中心思想(controlling idea)。中心思想的作用是導向(control)和制約(limit)。我們前面談到的關鍵詞就反映了中心思想。所謂導向就是規定段落的發展脈絡，所謂制約就是限制主題的覆蓋范圍，兩者不可分割。沒有導向，內容就會離題或偏題;沒有制約，內容就可能超出一個段落所能容納的范圍。
對于初學寫作的人來說，“導向”的重要性容易理解，但如果由他們自己來構思一個主題句，就可能忽視“制約”這一面。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.
本段的主題句如果沒有in which 引出的定語從句，那么two ways 不僅不能起制約作用，而且也不能起導向作用，句子本身讀起來也就使人覺得欠缺點什么。
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"這是毫無疑義的。但主題句中如不加上"to your heart"來加以 control 和 limit，那就流于空泛。
因此，緊緊把握主題句中controlling idea的導向和制約作用，是短文寫作成功關鍵之一。
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