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。