1．什么是主题句？主题句（topic sentence）是表达段落主题的句子。
它用以概括段落大意，要求全段其他文字都围绕它展开。
请看下例：My mother has passed along to me certain rules for getting along with others. Don't argue with parents; they will think you don't love them. Don't argue with children; they will think themselves victimized. Don't argue with spouses; they will think you are a tiresome mate. Don't argue with strangers; they will think you are not friendly. My mother's rules, in fact, can be summed up in two words: Don't argue.Our life today depends very much on energy. In towns and in villages, on farms and in factories, machines have made life easier than it used to be. The machines use energy, and energy is needed for heating, lighting, communications, carrying goods -- everything. Factories and industrial plants use a great deal of energy to make the things that we use and buy and sell.2 如何写好主题句？（1）写好主题句中的关键词关键词要尽量写得具体些。
对“具体”的要求包括两个方面：一是要具体到能控制和限制段落的发展；二是要具体到能说明段落发展的方向。
原句1: He can fix a bicycle himself.斜体部分应为主题句中的关键词。
但它只是限制了段落发展的部分内容，并没有告诉读者该用哪种方法展开，是用因果关系法还是用分类法？修正：He can fix a bicycle himself in several simple steps. 修正：There are several reasons why he can fix a bicycle himself.原句2: She tries to improve her looks.斜体部分应为主题句中的关键词。
她试着改变她面容的理由是什么？或者她采取什么方法来改善呢？我们无法从关键词中清楚得知。
修正：She tries many ways to improve her looks.修正：There are some reasons why she tries to improve her looks。
（2）写好主题句的中心思想Exercise is beneficial to your heart. A 22-year study was conducted by doctors in California. They found that people who work at physical jobs experience fewer heart attacks than other people. These active people work all thetime at moderate speeds, their daily routine gives them an adequate amount of exercise and helps them stay in shape."Exercise is beneficial"这是毫无疑义的。
但主题句中如不加上"to your heart"来加以control 和limit，那就流于空泛。
因此，紧紧把握主题句中controlling idea的导向和制约作用，是短文写作成功关键之一。
（3）要注意主题的可写性所定的主题不能太笼统，也不能太具体。
In Hemingway's story “Soldier’s Home”，Krebs tells his mother that he wants no part of God's Kingdom．这个句子太具体以至于无东西可写，无法展开段落。
In Hemingway's story “Soldier’s Home”，Kreb is a lonely，cynical veteran．这个句子可作为主题句，因为可以围绕“lonely，cynical”展开段落。
Let me tell you something about overpopulation．这个句子太笼统，作为一个段落的主题太大，难于驾驭。
Why has the world population been increasing so rapidly in the last decades？这个句子是个较好的主题句，作者用“the reasons for overpopulation in the last decades”，从“原因”（cause）和“时间”（time）两方面限制了主题，能在一个段落内论述。
2．推展句2.1 主要推展句主要推展句(major supporting statement)的主要特点是：围绕段落主题句展开的每一个推展句本身都不要求作进一步的说明或证明，句与句之间的关系是相互独立又是互相连接的。
例1：(主题句) There are several factors affecting climate. (推展句1)One factor is the mount of sunlight received. (推展句2)Altitude, or the height above sea level, also determines climate. (推展句3)The oceans have an effect on the climate of adjacent land. (推展句4)In addition, climate is influenced by the general circulation of the atmosphere.主题句指出影响气候的几个因素。
然后用四个扩展句说明四种因素。
第一种是太阳光的接收量，第二种是海拔高度，第三句和第四句分别是海洋和大气环流因素。
2.2 次要推展句次要推展句(minor supporting statement)是指对主要推展句作进一步的事实分析和举例说明。
它从属于某一个或某几个推展句。
例2：(主题句)I don't teach because teaching is not easy for me. (主要推展句1)Teaching is the most difficultof the various ways I have attempted to earn my living: mechanic, carpenter, writer. (主要推展句2)For me, teaching is a red-eye, sweaty-palm, sinking-stomach profession. (次要推展句1)Red-eye, because I never feel ready to teach no matter how late I stay up preparing. (次要推展句2)Sweaty-palm, because I'm always nervous before I enter the classroom, sure that I will be found out for the fool that I am. (次要推展句3)Sinking-stomach, because I leave the classroom an hour later convinced that I was even more boring than usual.从属于主要推展句2的三个次要推展句起着解释说明作用，分别解释red-eye, sweaty-palm, sinking-stomach的含义，这就更加形象生动地证明了"teaching is not easy" 这个主题。
Directions: For this part, you are allowed 30 minutes to write a composition on the topic Credit Cards on Campus. You should write at least 150 words, and base your composition on the outline (given in Chinese) below:1)近年来越来越多的大学生开始使用信用卡2)分析产生该现象的原因3)针对大学生使用信用卡提出你的建议Credit Cards on CampusIn recent years, credit cards have gained more popularity among college students. With banks’more focus on college market, an increasing number of college students have applied for credit cards and begun to consume by them.The following three factors can account for the popularity of credit cards on campus. Firstly, a credit card provides overdrawing service, which is a big attraction for those students who have not much money for their domination. Secondly, a credit card can help release financial burdens on those poor students, and hence reduce the risk of their dropping school for lacking money. Besides, many banks make some preferential policies on credit cards, like sending a gift, lowering year cost. Drawnby these policies, many college students have applied for credit cards.As far as I am concerned, it is not a bad thing for college students to own credit cards. However, there is misuse of credit cards among college students. Many students use credit cards to buy luxurious things they don’t need really, and even some of them owe a big credit card debt. Therefore, schools and banks have responsibility to let college students learn how to use credit cards properly. Only in this way can college students enjoy really the advantages of credit cards.段落基本要素好的文章有一些主要的构成要素，段落就是其中之一。