一般来说,一个好的段落应该具有这样的特点:意思完整，主题一致,前后连贯,过渡自然。换句话说，一个好的段落必须能够充分地表达思想，内容上没有任何残缺;一个段落只有一个主题，所有的扩展句都为主题句服务,主题句与扩展句在意义上始终保持统一;段落结构安排合理、条理清楚，句与句之间的转换自然、顺理成章，所有这些都要求段落在组织时要坚持以下三个原则： 

(一）统一性 

一个段落应该只说明一个问题，或一个问题的某一方面;应该只叙述一件事情，或一件事情的某一个阶段。也就是说，一个段落内的各个句子必须服从于一个中心，任何游离于中心思想以外的句子都是不可取的。 一个好的段落应该具有统一性。统一性就是一个中心思想统领整个段落。每个细节都应支持主题句，不应有无关的细节。为了支持主题，要尽可能多地提供细节，但又要避免滥用细节。请看下面的例子：

 

Millions of years ago，Australia was linked to Asia by a land bridge. Then an earthquake caused the land bridge to sink. The sea rushed in, and Australia became an island. Later, this island became a British colony. Many animals that once wandered back and forth across the land bridge were left in Australia. And kangaroos were among them. Though they were killed off by large animals in other places，they were about to survive in Australia.

 

这一段讲的是远古时期的澳大利亚，当时连人类都没有，但其中画线的句子偏离主题，应当被删掉。又如：

 

Learning a foreign language has changed. Not long ago, students would sit with pen in hand, writing the basic forms of a language, learning structures they would never be able to speak. In that same classroom today, pens and notebooks have been put away. The spoken sounds of foreign tongue fill the room. Today the last skill learned writing a foreign language comes as a natural and possible part of the total language learning process. Yet, just a few years ago, the last skill learned was the first skill mastered today—speaking a foreign tongue. To visit some foreign countries, travelers have to apply for visas.

此段落每句均围绕主题句“Learning a foreign language has changed. ”展开，但其中画线的一句“To visit some foreign countries，travelers have to apply for visas. ”则是说申请护照的，明显地不符合此段主题， 所以应该删除。

 

(二）完整性 

 

一个段落的主题思想要靠扩展句来实现，如果只有主题句而没有扩展句来进一步交代和充实，就不能构成一个完整的段落。同样，如果只有扩展句，但主题思想没有得到相对圆满的交代，给读者一种意犹未尽的感觉，这样的段落也不能完成其交际功能。请看下面段落：

Physical work can be a useful form of therapy for a mind in turmoil. Work concentrates your thoughts on a concrete task. Besides, it is more useful to work—you produce something rather than more anxiety or depression.

本段的主题句是段首句，其后两个扩展句均不能回答主题句所提出的问题，什么是a mind in turmoil? physical work又如何能改变这种情况？为什么它能起therapy的作用？读者得不到正确答案。 又如下面一例：

In my English study, I have many difficulties. My first difficulty is that my vocabulary is not large. The second difficulty I have is that I cannot write well. The third difficulty is that my listening comprehension is not good enough. Now I have a plan to study English well.

此段落看似完整，第一句是主题句，后文提到的“my first difficulty, the second difficulty, the third diffuuilty”分别为扩展句，最后一句为结尾句。此段落虽然有头有尾，但缺少实质内容——没有分析学英语困难的原因，使得文章空洞无物。此外，结尾句提到了英语学习计划，具体内容却没有交代，给人留下话好像没说完的感觉。我们比较一下修改补充之后的效果：

 

In my English study, I have many difficulties. The following are some of them. My first difficulty is that my vocabulary is not large. As a result, I can not read fast or understand well. As I am coming to middle age, it is not so easy to keep many new words in mind quickly. The second difficulty I have is that I cannot write well. As I have been very busy with my work which really doesn’t need any foreign language, I seldom have a chance to write something in English. The third difficulty is that my listening comprehension is not good enough. This is because I had little training in listening while I was at college.