在这个段落中，句（1）是主题句，提出计算机在所有领域都有应用的观点；句（2）、（3）和（4）是一级扩展句，都支持和说明句（1），具体说明计算机都在哪些领域应用，而句（5）是二级扩展句，用来补充说明句（4），通过举例来说明计算机辅助设计的用途。

(1)In rock music there is a distinct and almost overwhelming beat. (2)No single beat is characteristic of the music today. （3）But each song has an easily recognizable rhythm. (4)As you listen to a song, your foot usually starts to pick up the beat. (5)Before long, your entire body seems to be moving with it. （6）Your head pounds with the beat, and there is no room for thought. （7）Only the surge of the music is important. (8)In its own way, rock music is as dominant as the rock Gibraltar. Its message is an overgrowing emotional one.

在这一段中，句（1）是段落主题句，点出本段主题思想，要说明“摇滚乐最突出的特点是其节奏”。句（2）、（3）是一级扩展句，对主题句进一步补充说明；而句（4）、（5）、（6）是二级扩展句，从属于句（2），对句（2）起着补充、解释和说明的作用，更形象、生动地证明了主题句所表达的思想。这里，句（7）是结尾句，它的功用在后面的章节里会做详尽的讲解。

He who mistrusts most should be trusted least.——最不信任别人的人最不应该得到信任。

三、结尾句

结尾句位于段落的末尾，在段落中也起着非常重要的作用，用以对全段内容进行总结、归纳或提出结论性的观点。结尾句常和主题句相呼应，以不同的形式再现主题，并与扩展句相关联。但是并非任何段落都必须有结尾句才算完整。有的段落，尤其是一些较短的段落中只有主题句没有结尾句。

请看下面各段落中结尾句的写法：

Scientists believe that there has been life of some sort on the earth for about twelve hundred million years. Such a large figure is difficult to grasp, so let us scale it down and reckon it as one hundred years. We then find that man has been on the earth for about a month, and civilized man for only seven or eight hours. So you see mankind is only at the beginning of its civilized life, and we must not expect too much.

本段的结尾句是对全段的总结归纳。它进一步强调了段落的中心思想，以引起读者的注意和重视。

A contract is an agreement which is enforceable by law. It can be long or short, formal or informal, simple or complicated, and verbal or written. Without a contract or agreement to bind the contracting parties, any international business or transaction would be impossible. Long age, people exchanged promises in making bargains and binding the conduct of others. This exchange of promises came to be known as “agreement” and became more and more important in the fields of business. A promise or agreement is reached as a result of the process of offer and acceptance. When an agreement is reached, a contract is formed. Once a contract is officially signed by the concerned parties, it creates legal obligations in the sense of law.

本段的最后一句是结尾句，第一句是主题句。结尾句所表达的意思跟主题句一致，但句子结构和用词都不一样。结尾句是主题句的再现，起着前后呼应的作用，段落中心思想更加突出明显。

The alarming problem is the increase in violent incidents owing to love on campus. When failing to win desirable affection from their lovers, some girl students tend to commit suicide while boys turn to knife and poison for revenge. Besides, it is a painful fact that boys obtain money needed for their love in an improper way. To please their girlfriends, boy students always dress smartly, shop generously and eat out frequently; but when finding no other quicker way to make the money needed than by stealing or cheating, they are on the way to imprisonment. Another sensitive problem is the growing pregnancy rate among girl students. The young students are subjected to blame for being too young to put themselves under sensible control, but school authorities cannot escape their share of responsibility.