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)。
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。
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.
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"?
