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：
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。
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.
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.