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. 　
　　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. 　
　　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，那就流于空泛本文摘 自教 育文'摘’liuxuepaper.com。
　　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。
2．推展句
2.1 主要推展句
　　主要推展句(major supporting statement)的主要特点是：围绕段落主题句展开的每一个推展句本身都不要求作进一步的说明或证明，句与句之间的关系是相互独立又是互相连接的。
　　例1：(主题句) There are several factors affecting climate. (推展句1)One factor is the mount of sunlight received. (推展句2)Altitude, or the height above sea level, also determines climate. (推展句3)The oceans have an effect on the climate of adjacent land. (推展句4)In addition, climate is influenced by the general circulation of the atmosphere.
　　主题句指出影响气候的几个因素。然后用四个扩展句说明四种因素。第一种是太阳光的接收量，第二种是海拔高度，第三句和第四句分别是海洋和大气环流因素。
2.2 次要推展句
　　次要推展句(minor supporting statement)是指对主要推展句作进一步的事实分析和举例说明。它从属于某一个或某几个推展句。
　　例2：(主题句)I don't teach because teaching is easy for me. (主要推展句1)Teaching is the most difficult of the various ways I have attempted to earn my living: mechanic, carpenter, writer. (主要推展句2)For me, teaching is a red-eye, sweaty-palm, sinking-stomach profession. (次要推展句1)Red-eye, because I never feel ready to teach no matter how late I stay up preparing. (次要推展句2)Sweaty-palm, because I'm always nervous before I enter the classroom, sure that I will be found out for the fool that I am. (次要推展句3)Sinking-stomach, because I leave the classroom an hour later convinced that I was even more boring than usual.liuxuepaper.com