一般来讲，任何一个段落无论长短都会涉及问题的一个方面，也就是说每个段落往往要围绕一个主题展开。如东一句，西一句，文章的脉络就不会清晰，读者也就无从抓住重点。段落无论长短都会有说明主题的句子，我们称它为主题句(Key Sentence / Topic Sentence)。抓住主题句就等于抓住了段落的中心。因而，准确判断并找出主题句相当重要。主题句一般会出现在段首或段尾，起概括全段的作用。主题句也可以出现在段中，同时还可以首尾呼应，段落结束时再次强调本段落的中心思想。请看下面的例子：

例1：阅读下面的段落：

Students today socialise in ways quite unlike those of their parents. One of the primary differences is the use and abuse of drugs. During the ‘60s and ‘70s, students were known to smoke the odd joint and occasionally take harder substances, whereas according to recent polls, students today are using ever larger quantities of drugs on an ever more regular basis. Whilst cannabis is still the drug of choice among young people, synthetic drugs such as ecstasy and harder drugs such as cocaine and even heroine are being taken by an increasing number of students. Drugs are much more readily available to students and are much cheaper than ever before.

段首句为主题句：“Students today socialize in ways quite unlike those of their parents.”它揭示了这段的中心思想是“现在的学生以一种不同于他们父母所采用的方式进行着社会活动。”余下的各个句子都是以具体的事例来支持说明这一中心论点的。

例2：阅读下面的段落：

We surveyed local residents in three university towns throughout the state to find out their opinions on the students in those towns. We questioned over three thousand people and found that since our last survey three years ago, attitudes towards students have changed considerably. “Lazy,” “scruffy” and “bad-mannered” was how more than 40% of those questioned described students, up from 22% in the last survey. This year only 25% considered students to be “polite” and “friendly” compared with 39% three years ago. Overall, 49% considered students to be a positive part of the town compared to 58% last time.

这次调查发现，与三年前的一次调查相比，人们对于学生的态度产生了相当大的变化。这是本段的主题。然后用具体的百分比数字说明这一中心观点。主题句在段落中间。

例3：阅读下面的段落：

Now let’s look at an interesting example with color. If you own a blue car, you know it is blue, even it is in bright sunlight, in darkness or under a yellow street light. Your eyes tell you that it changes color in these different situations, but your brain knows better. You have learnt that cars don’t change color all time, and so your brain interprets the information your eyes give you.

主题句在段尾：“你的大脑在解释你的眼睛传递给它的信息。”前面有关蓝色汽车的例子正是为说明这一观点服务的。

主题句固然可以很好地向读者说明段落的中心思想，但阅读中我们常常会发现，有时很难在具体的位置找到一个明显的能说明主题的句子。这时就需要我们对段落的整体内容进行归纳总结，高度概括提炼出它的中心思想。

注意到了主题句的位置，抓住了主题句，就等于抓住了段落的中心思想。除此之外，阅读时还应注意一些进一步归纳总结的词汇和短语的使用以及其他一些措辞，因为这些词的使用可以就什么是主题为读者提供有用的线索。