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 thetopic)，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(completenessoradequateness)。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)。
本段的主题句是段首句，controlling idea(中心思想)是take the long trip across thecountry。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 retirementdinner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 irrelevantsentences(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 twohundred pounds, but I joined the YMCA for an exercise class and diet program. Inone year I lost eighty pounds. I feel much better and never want to have thatmuch weight on my five-feet frame again. I bought two new suitcases last week.Everyday I practice jogging three miles, swimming fifteen laps, liftingtwenty-pound weights and playing tennis for one hour. My mother was a prematurebaby.
本段的controlling idea 是like to deep physically fit，但段中有两个irrelevantsentences，一个是I bought two new suitcases last week，另一个是My mother was a prematurebaby。
Physical work can be a useful form of therapy for a mind in turmoil. Workconcentrates your thoughts on a concrete task. Besides, it is more useful towork ---- 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)Physicalwork又怎样能改变这种情况?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 picture is worth a thousand words. Sometimes,pictures are pretty useless things. If you can''t swim and fall in the river andstart gulping water, will you be better off to hold up a picture of yourselfdrowning, or start screaming "Help"?
We had a number of close calls that day. When we rose, it was obviouslylate and we had to hurry so as not to miss breakfast; we knew the dining roomstaff was strict about closing at nine o''clock. Then, when we had been drivingin 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 ofour drinking water to cool it down. By the time we reached the mountain, it wasfour o''clock and we were exhausted. Here, judgement ran out of us and we startedthe tough climb to the summit, not realizing that darkness came suddenly in thedesert. Sure enough, by six we were struggling and Andrew very nearly went downa steep cliff, dragging Mohammed and me along with him. By nine, when the windhowled across the flat ledge of the summit, we knew as we shivered together forwarmth 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 nineo''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, wecould see it flesh out before our eyes. It was tubular, all right, but fatterthan we could see from far away. Furthermore, we were also astonished to noticethat the building was really in two parts: a pagoda sitting on top of a tubularone-story structure. Standing ten feet away, we could marvel at how much of thepagoda was made up of glass windows. Almost everything under the wonderfulChinese roof was made of glass, unlike the tube that it was sitting on, whichonly had four. Inside, the tube was gloomy, because of the lack of light. Then asteep, narrow staircase took us up inside the pagoda and the light changeddramatically. All those windows let in a flood of sunshine and we could see outfor 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 inexpressing yourself to be effective. If you work on a machine, your ability toexpress yourself will be of little importance. But as soon as you move one stepup from the bottom, your effectiveness depends on your ability to reach othersthrough the spoken or the written word. And the further away your job is frommanual work, the larger the organization of which you are an employee, the moreimportant it will be that you know how to convey your thoughts in writing orspeaking. In the very large business organization, whether it is the government,the large corporation, or the Army, this ability to express oneself is perhapsthe 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 beencareful enough to keep him on the path. This carelessness can take any number offorms. Perhaps a sentence is so excess.