要寫好一個段落，第一步就是必須要決定你寫作的目的 (purpose)。
就像前面所說的，一個段落應該是「能表現出一個獨立的思想」的單元。因此，在開始寫作前你就要先決定好文章每一個段落所要闡述的 idea 是什麽。
一旦確定好了 idea 之後・下一步就是要使讀者能夠很清楚知道這個 idea 是什麼？而「主句」(topic sentence) 的作用就在於此。(大家可以回去參考前兩個單元的範例文章，找一下段落中的主句)
主句的寫法是很重要的，不能夠寫得太籠統 (too general)，因為主句也是用來幫助你專注在真正的主題上，限制一下自己的 idea 而不要越寫越偏離主題。主句中通常也會展現出段落中的重要概念，也就是 controlling idea。以下舉幾個主句的例子給大家看：
例一：
2001: A space Odyssey is an interesting movie. (太籠統)
2001: A space Odyssey describes some of the problems of space travel. (具體指出主題)
例二：
The Olympic Games are exiting. (太籠統)
In the Olympic Games the athletes of many nations compete intensely. (具體指出主題)
從上面的例子，有沒有感覺到主句該有的樣子，以及包含在其中的 controling idea (在上面用黑體字標示的) 是什麼呢？
討論：
有了這樣的概念後，我們來看幾段範例的段落，讀完之後請練習分析一下，這些段落是否符合：1. Unity 2. Coherent? 3. 請找出 Topic sentence 以及段落的 controlling idea。
範例段落 1
Easterners commonly complain that there is no "weather" at all in Southern California, that the days and the seasons slip by relentlessly numbingly bland. That idea is quite misleading. In fact the climate is characterized by infrequent but violent extremes: two periods of torrential subtropical rains which continue for weeks and wash out the hills and send subdivisions sliding toward the sea, and about twenty scattered days a year of the Santa Ana [wind), which, with its incendiary dryness, invariably means fire. At the first prediction of a Santa Ana, the Forest Service flies men and equipment from northern California into the southern forests, and the Los Angeles Fire Department cancels its ordinary non-firefighting routines. The Santa Ana caused Malibu to burn the way it did in 1956, and Bel Air in 1961, and Santa Barbara in 1964. In the winter of 1966-67 eleven men were killed fighting a Santa Ana fire that spread through the San Gabriel Mountains
摘自 Joan Didion, Slouching Towards Bethlehem
範例段落 2
In some of the Quechua [language] of Peru and Bolivia one speaks of the future as"behind oneself "and the past as "ahead." Such interpretations of time have given rise to remarks by foreigners that the Quechuas have "a perverted philosophical instinct." However, the Quechuas argue "If you try to see the past and future with your mind's eye, which can you see? " The obvious answer is that we can "see" the past and not the future, to which the Quechua replies, "Then, if you can see the past, it must be ahead of you; and the future, which you cannot see, is behind you." Such an explanation does not mean that the Quechuas worked out a philosophical interpretation of the past and future before talking about it, but it does suggest that there may be equally valid but opposite ways of describing the same thing.
摘自 "How the Quechuas Think, "Eugene Nida, Customs and Cultures
請注意，主句不一定在段落的第一行喔！