文章的正文应以文章的开头为线索，具体地叙述、说明或论证文章的主题。文章不论长短，每个段落都必须为主题服务。像说明文和议论文这一类的文章，一个主题还常分成几个小主题，每个小主题要用一个段落处理，另起一段时，应是一层新的意思。每一段的开头，要放一个表示段落小主题的主题句，这样可使文章条理化，易于阅读，便于读者抓住主题。段内的所有句子应围绕主题句的意义加以阐述或论证，为中心思想服务。句子之间应衔结自然，有条不紊，而且还要合乎逻辑，段落中不能出现任何与主题无关的句子；英语写作比较重视主题句的作用，缺少它段落意义就会含糊不清。主题句也可放在段落的中间和末尾等部位，但对初学者来说，以放在段首为好。见下列这篇题为"How to Be a Good Student" (怎样做个好学生)的文章：

　　We students are the builders and masters of the country. It is important for us to know how to be a good student.

　　A good student, I think, should be diligent in his studies. The more he studies, the more he will increase his knowledge. Without enough knowledge, we cannot make great contributions to the modernization of our country.

　　To take care of one`s own body is another important thing for good student to do. Anyone, who hasn`t got a strong body, can do nothing for his country, even if he has much knowledge. There was a man, who, when he was student, studied hard but neglected his health. No sooner did he come to serve the country than he died of poor health. From this we may see that to have a strong body is really very important for a student.

　　Lastly, to cultivate one`s own virtue is most important. Virtue is the essence
of a noble and good character. It will greatly help one to be useful and his country heart and soul. When learned people go astray, they do more harm than good to society. We should draw lessons from this.

这篇文章的第一段引出了文章的主题，第二、第三和第四段则是文章的正文，每—段的第一句即是段落的主题句，它们既支持了文章中心的观点和思想，同时又概括了全段的意思。在同一段落中，其余的句子都围绕主题句所表示的中心展开，同时句子间的衔结也很自然；各层的意思都很连贯。

　　分段是文章组织上重要的一步，但如果写的题目范围很小，那就无须再将题目分成小的主题，并分入各个段落去阐述了。像一篇简短的评论；某一事情的简短记载，某一个想法的说明，对一个人物或一件事情的简要叙述或说明等，就可以只用一个段落来表现主题。

　　在记叙文中，段的结构有时可以很简单，不需要有主题句，叙事一气呵成，中途没有停顿。段与段之所以分开，只是为了起修辞作用，以便把某一细节置于显著的地位。