1.統(tǒng)一性

一個段落內的各個句子必須從屬于一個中心，任何游離于中心思想之外的句子都是不可取的。請看下例：

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.

本段的主題句是段首句，controlling idea(中心思想)是take the long trip across the country。文中出現(xiàn)兩個irrelevant sentences，一個是Bella bakes the best rhubarb pie，這一段是講的是Joe and I，中間出現(xiàn)一個Bella是不合適的。還有，Joe received a watch at his retirement dinner這一句更是與主題句不相關。考生在作文卷上常常因為造出irrelevant sentences(不相關語句)而丟分，值得引起注意。再看一個例子：

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，但段中有兩個irrelevant sentences，一個是I bought two new suitcases last week，另一個是My mother was a premature baby。

從上面兩個例子可以看出，native speakers同樣會造出來irrelevant sentences。卷面上如果這種句子多了，造成偏題或離題，那問題就更嚴重了。

2.完整性

正象我們前面說得那樣，一個段落的主題思想靠推展句來實現(xiàn)，如果只有主題句而沒有推展句來進一步交待和充實，就不能構成一個完整的段落。同樣，雖然有推展句，但主題思想沒有得到相對圓滿的交待，給讀者一種意猶未盡的感覺。這樣的段落也不能完成其交際功能。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.

本段的主題句是段首句。本段的兩個推展句均不能回答主題句中提出的問題。什么是“a mind in turmoil”(心境不平靜)Physical work又如何能改變這種情況？為什么它能起therapy的作用？讀者得不到明確的答案。

若要達到完整就必須盡可能地簡明。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.

段首句所表達的主題思想是一種看法，必須有具體事例加以驗證。上述兩個推展句只是在文字上對主題作些解釋，整個段落內容空洞，簡而不明。如果用一兩個具體的例子的話，就可以把主題解釋清楚了。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"?