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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。
1.2 如何写好主题句中的关键词
段落的主题句对主题的限定主要是通过句中的关键词来表现的。关键词要尽量写得具体些。对“具体”的要求包括两个方面：一是要具体到能控制和限制段落的发展；二是要具体到能说明段落发展的方法。准确地把握关键词是清楚地表达段落主题、写好段落主题句的重要前提之一。在上面的例1，例2中，主题句的关键词分别为：depends very much on energy; no one can avoid being influenced. 我们再看下列例句：
原句1: he can fix a bicycle himself。
斜体部分应为主题句中的关键词。但它只是限制了段落发展的部分内容，并没有告诉读者该用哪种方法展开，是用因果关系法还是用分类法？
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。
斜体部分应为主题句中的关键词。她试着改变她面容的理由是什么？或者她采取什么方法来改善呢？我们无法从关键词中清楚得知。
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 如何写好主题句的中心思想
主题句由两部分组成，即主题(topic)和中心思想(controlling idea)。中心思想的作用是导向(control)和制约(limit)。我们前面谈到的关键词就反映了中心思想。所谓导向就是规定段落的发展脉络，所谓制约就是限制主题的覆盖范围，两者不可分割。没有导向，内容就会离题或偏题；没有制约，内容就可能超出一个段落所能容纳的范围。
对于初学写作的人来说，“导向”的重要性容易理解，但如果由他们自己来构思一个主题句，就可能忽视“制约”这一面。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。
本段的主题句如果没有in which 引出的定语从句，那么two ways 不仅不能起制约作用，而且也不能起导向作用，句子本身读起来也就使人觉得欠缺点什么。
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"这是毫无疑义的。但主题句中如不加上"to your heart"来加以 control 和 limit，那就流于空泛。
因此，紧紧把握主题句中controlling idea的导向和制约作用，是短文写作成功关键之一。