首先，一个段落必须有一个中心即主题思想，该中心由主题句特别是其中的题旨来表达。整个段落必须 紧扣这个主题(stick or hold to the topic)，这就是段落的统一性(unity)。其次，一个段落必须有若 干推展句，使主题思想得到充分展开，从而给读者一个完整的感觉，这就是完整性(completeness or adequateness)。再者，一个段落不是杂乱无章的，而是有机的组合，句子的排列顺序必须合乎逻 辑，从一个句子到另一个句子的过渡必须流畅(smooth)，这就是连贯性(coherence)。下面我们就对 这三个标准分别加以说明。
Joe and I decided to take the long trip we'd always wanted across the country. We were like young kids buying our camper and stocking it with all the necessities of life. Bella bakes the best rhubarb pie. We started out in early spring from Minneapolis and headed west across the northern part of the country. We both enjoyed those people we met at the trailer park. Joe received a watch at his retirement dinner. To our surprise, we found that we liked the warm southern regions very much, and so we decided to stay here in New Mexico.
本段的主题句是段首句，controlling idea（中心思想）是take the long trip across the country。文中出现两个irrelevant sentences，一个是Bella bakes the best rhubarb pie，这 一段是讲的是Joe and I ，中间出现一个Bella是不合适的。还有，Joe received a watch at his retirement dinner这一句更是与主题句不相关。考生在四级统考的作文卷上常常因为造出 irrelevant sentences（不相关语句）而丢分，值得引起注意。再看一个例子：
My name is Roseanna, and I like to keep physically fit. I used to weigh two hundred pounds, but I joined the YMCA for an exercise class and diet program. In one year I lost eighty pounds. I feel much better and never want to have that much weight on my five-feet frame again. I bought two new suitcases last week. Everyday I practice jogging three miles, swimming fifteen laps, lifting twenty-pound weights and playing tennis for one hour. My mother was a premature baby.
本段的controlling idea 是like to deep physically fit，但段中有两个irrelevant sentences，一个是I bought two new suitcases last week，另一个是My mother was a premature baby。
从上面两个例子可以看出，native speakers同样会造出来irrelevant sentences。卷面上如 果这种句子多了，造成偏题或离题，那问题就更严重了。
正象我们前面说得那样，一个段落的主题思想靠推展句来实现，如果只有主题句而没有推展句来进 一步交待和充实，就不能构成一个完整的段落。同样，虽然有推展句，但主题思想没有得到相对圆满的 交待，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。例如：
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。本段的两个推展句均不能回答主题句中提出的问题。什么是 ＂a mind in turmoil＂（心境不平静）Physical work又如何能改变这种情况？为什么它能起therapy的作用？读 者得不到明确的答案。
It is not always true that a good picture is worth a thousand words. Often writing is much clearer than a picture. It is sometimes difficult to figure out what a picture means, but a careful writer can almost always explain it.
段首句所表达的主题思想是一种看法，必须有具体事例加以验证。上述两个推展句只是在文字上对 主题作些解释，整个段落内容空洞，简而不明。如果用一两个具体的例子的话，就可以把主题解释清楚 了。比如下段：
It is not always true that a picture is worth a thousand words. Sometimes, pictures are pretty useless things. If you can't swim and fall in the river and start gulping water, will you be better off to hold up a picture of yourself drowning, or start screaming "Help"?
连贯性包括意连和形连两个方面，前者指的是内在的逻辑性，后者指的是使用转换词语。当然这两 者常常是不可分割的。只有形连而没有意连，句子之间就没有内在的有机的联系；反之，只有意连而没 有形连，有时行文就不够流畅。
We had a number of close calls that day. When we rose, it was obviously late and we had to hurry so as not to miss breakfast; we knew the dining room staff was strict about closing at nine o'clock. Then, when we had been driving in the desert for nearly two hours ----- it must have been close to noon ---- the heat nearly hid us in; the radiator boiled over and we had to use most of our drinking water to cool it down. By the time we reached the mountain, it was four o'clock and we were exhausted. Here, judgement ran out of us and we started the tough climb to the summit, not realizing that darkness came suddenly in the desert. Sure enough, by six we were struggling and Andrew very nearly went down a steep cliff, dragging Mohammed and me along with him. By nine, when the wind howled across the flat ledge of the summit, we knew as we shivered together for warmth that it had not been our lucky day.
本段从 ＂rose＂（起床）写起，然后是吃早餐(＂not to miss breakfast＂, ＂closing at nine o'clock＂)，然后是 ＂close to noon＂，一直写到这一天结束(＂By nine--")。
B. 按位置远近排列(spatial arrangement)。例如：
From a distance, it looked like a skinny tube, but as we got closer, we could see it flesh out before our eyes. It was tubular, all right, but fatter than we could see from far away. Furthermore, we were also astonished to notice that the building was really in two parts: a pagoda sitting on top of a tubular one-story structure. Standing ten feet away, we could marvel at how much of the pagoda was made up of glass windows. Almost everything under the wonderful Chinese roof was made of glass, unlike the tube that it was sitting on, which only had four. Inside, the tube was gloomy, because of the lack of light. Then a steep, narrow staircase took us up inside the pagoda and the light changed dramatically. All those windows let in a flood of sunshine and we could see out for miles across the flat land.
本段的写法是由远及近，从远处(＂from a distance＂)写起，然后＂get closer＂，再到(＂ ten feet away＂)，最后是 ＂inside the pagoda＂……当然，按位置远近来写不等于都是由远及 近。根据需要，也可以由近及远，由表及里等等。
C. 按逻辑关系排列(logical arrangement)
a. 按重要性顺序排列(arrangement in order of importance)
If you work as a soda jerker, you will, of course, not need much skill in expressing yourself to be effective. If you work on a machine, your ability to express yourself will be of little importance. But as soon as you move one step up from the bottom, your effectiveness depends on your ability to reach others through the spoken or the written word. And the further away your job is from manual work, the larger the organization of which you are an employee, the more important it will be that you know how to convey your thoughts in writing or speaking. In the very large business organization, whether it is the government, the large corporation, or the Army, this ability to express oneself is perhaps the most important of all the skills a man can possess.
这一段谈的是表达能力，它的重要性与职业，身份有关，从＂not need much skill"或 "of little importance"到 "more important"，最后是 "most important"。
If a reader is lost, it is generally because the writer has not been careful enough to keep him on the path. This carelessness can take any number of forms. Perhaps a sentence is so exces