英语写作是学习英语的重要课题之一。它既是英语学习短期目标??考试所要求的内容之一；又是英语学习的长期目标??交流所必需的能力之一。英语写作是集基础知识，基本能力和课外信息于一身的综合体现，同时，又要兼顾到写作者的写作技巧。因而，想写出一篇优秀的作文需要有扎实的英语语法知识、长期不懈的训练、、科学的方法指导和广博的课本以外的知识和信息。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）。
那么下面就先举例说说段落的“统一性”。
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(不相干的句子)，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。
