一般來講，任何一個段落無論長短都會涉及問題的一個方面，也就是說每個段落往往要圍繞一個主題展開。如東一句，西一句，文章的脈絡就不會清晰，讀者也就無從抓住重點。段落無論長短都會有說明主題的句子，我們稱它為主題句(Key Sentence / Topic Sentence)。抓住主題句就等于抓住了段落的中心。因而，準確判斷并找出主題句相當重要。主題句一般會出現在段首或段尾，起概括全段的作用。主題句也可以出現在段中，同時還可以首尾呼應，段落結束時再次強調本段落的中心思想。請看下面的例子：

例1：閱讀下面的段落：

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.

段首句為主題句：“Students today socialize in ways quite unlike those of their parents.”它揭示了這段的中心思想是“現在的學生以一種不同于他們父母所采用的方式進行著社會活動。”余下的各個句子都是以具體的事例來支持說明這一中心論點的。

例2：閱讀下面的段落：

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.

這次調查發現，與三年前的一次調查相比，人們對于學生的態度產生了相當大的變化。這是本段的主題。然后用具體的百分比數字說明這一中心觀點。主題句在段落中間。

例3：閱讀下面的段落：

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.

主題句在段尾：“你的大腦在解釋你的眼睛傳遞給它的信息。”前面有關藍色汽車的例子正是為說明這一觀點服務的。

主題句固然可以很好地向讀者說明段落的中心思想，但閱讀中我們常常會發現，有時很難在具體的位置找到一個明顯的能說明主題的句子。這時就需要我們對段落的整體內容進行歸納總結，高度概括提煉出它的中心思想。

注意到了主題句的位置，抓住了主題句，就等于抓住了段落的中心思想。除此之外，閱讀時還應注意一些進一步歸納總結的詞匯和短語的使用以及其他一些措辭，因為這些詞的使用可以就什么是主題為讀者提供有用的線索。