1、统一性
一个段落内的各个句子必须从属于一个中心，任何游离于中心思想之外的句子都是不可取的。请看下例：
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。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。
2、完整性
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。因此，要达到完整就必须尽可能地简明。例如：
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"?
3、连贯性（coherence）
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。
1）、意连
段落中句子的排列应遵循一定的次序，不能想到什么就写什么。如果在下笔之前没有构思，边写边想，写写停停，那就写不出一气呵成的好文章来。下面介绍几种常见的排列方式。
A．按时间先后排列（chronological arrangement）
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 our 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”。
b.由一般到特殊排列（general-to-specific arrangement）
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 excessively cluttered that the reader, hacking his way through the verbiage, simply doesn't know what it means. Perhaps a sentence has been so shoddily constructed that the reader could read it in any of several ways. Perhaps the writer has switched tenses, or has switched pronouns in mid-sentence, so the reader loses track of when the action took place or who is talking. Perhaps sentence B is not logical sequel to sentence A —— the writer, in whose head the connection is clear, has not bothered to provide the missing link. Perhaps the writer has used an important word incorrectly by not taking the trouble to look it up. He may think that "sanguine" and "sanguinary" mean the same thing, but the difference is a bloody big one. The reader can only infer what the writer is trying to imply.
这一段谈的是a writer's carelessness，先给出一个general statement作为主题句，然后通过5个 ”perhaps”加以例证。
c. 由特殊到一般排列（specific-to-general arrangement）
I do not understand why people confuse my Siamese cat, Prissy, with the one I had several years ago, Henry. The two cats are only alike in breed. Prissy, a quiet, feminine feline, loves me dearly but not possessively. She likes to keep her distance from people, exert her independence and is never so rude as to beg, lick, or sniff unceremoniously. Her usual posture is sitting upright, eyes closed, perfectly still. Prissy is a very proper cat. Henry, on the other hand, loved me dearly but possessively. He was my shadow from morning till night. He expected me to constantly entertain him. Henry never cared who saw him do anything, whether it was decorous or not, and he usually offended my friends in some way. The cat made himself quite comfortable, on the top of the television, across stranger's feet or laps, in beds, drawers, sacks, closets, or nooks. The difference between them is imperceptible to strangers.
本段的主题句是段首句，它仅提出一个问题：为什么两只猫会被搞混。然后对两者进行比较，末句才下结论。
2）、形连
行文的逻辑性常常要靠适当的转换词语及其他手段来实现。请读下面这一段文字并找出文中用以承上启下的词语：
Walter's goal in life was to become a successful surgeon. First, though, he had to get through high school, so he concentrated all his efforts on his studies —— in particular, biology, chemistry, and math. Because he worked constantly on these subjects, Walter became proficient in them; however, Walter forgot that he needed to master other subjects besides those he had chosen. As a result, during his junior year of high school, Walter failed both English and Latin. Consequently, he had to repeat these subjects and he was almost unable to graduate on schedule. Finally, on June 6, Walter achieved the first step toward realizing his goal.
本文中起承上启下的词语有两种，一种是转换词语（transitional words or phrases），另一种是起转换作用的其他连接手段（linking devices）。前者依次有：first, though, so, in particular, and, because, however, besides, as a result, both…and, consequently, and, finally.后者依次是：he, he, his, his, he, these, them, he, those, his, he, these, his. 本段中共有词汇105个，所使用的转换词语及其他连接用语共26个词，约占该段总词汇量的四分之一。由此可见，掌握好transitions不仅对行文的流（smoothness）有益，而且对于学生在半个小时内写120个词也是不无好处的。
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