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首先，一個段落必須有一個中心即主題思想，該中心由主題句特別是其中的題旨來表達。整個段落必須緊扣這個主題（stick or hold to the topic），這就是段落的統一性（unity）。其次，一個段落必須有若干推展句，使主題思想得到充分展開，從而給讀者一個完整的感覺，這就是完整性（completeness or adequateness）。再者，一個段落不是雜亂無章的，而是有機的組合，句子的排列順序必須合乎邏輯，從一個句子到另一個句子的過渡必須流暢（smooth），這就是連貫性（coherence）。下面我們就對這三個標準分別加以說明。
1、統一性
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。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。
