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.托福考试成绩查询

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。