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)。下面我们就对这两个标准分别加以说明......
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。考生在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。
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.
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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。
若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"?
雅思写作频道编辑感谢您阅读《如何保证段落的完整性与统一性》一文.