一篇文章或一个段落通常都是围绕一个中心意思或者某一个主旨展开的，而这个中心意思通常可以用一个句子来概括。这个能概括文章或段落中心意思的句子叫做主题句。它能在文章中起到“画龙点睛”的作用，整个文章或者整个段落剩余的内容则是对主题句所提出的观点进行解释，扩展。抓住主题句就等于抓住了段落的中心。因此，理解一个段落或一篇文章的中心意思或者主旨，首先要学会寻找主题句。通常情况下，主题句的表现形式主要有三种：
(1)主题句在段首
主题句在段首或篇首的情况相当普遍，概括整个段落的主要内容。一般新闻报道、说明文，议论文大都采用先总述，后分述的文章结构。
例：
Students today socialize 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。
(2)主题句在段尾
用归纳法写文章时，往往表述细节的句子在前，概述性的句子在后，并以此结尾。这种位于段尾的主题句往往是对前面细节的总结，归纳或者再次强调本段的中心思想。
例：
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。
(3)主题句在段落中间
某些情况下会发现段首或者段落是在举例子或者是细节性描述，而段落中间存在概述性的一句话， 那么这句话便是整个段落的主题句，起到首尾呼应的作用。
例：
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.
主题句在段落中间：“since our last survey three years ago, attitudes towards students have changed considerably”它揭示了这段的主要说的是这次调查发现，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。然后用具体的百分比数字说明这一中心观点。
以上三种情况是主题句常出现的位置，但是