雅思大作文要求在40分钟内完成250字的写作，对于大部分学生来讲很难高质量地完成这个写作任务。造成这个问题的主要原因是考生不知道如何提出观点，然后进行论证。雅思大作文一般分为开头段、主体段和结论段。其中，开头段和结论段所占字数较少，而且写起来相对容易。而主体段字数较多，写起来要复杂的多。可以说，主体段的写作直接关系到能否取得比较理想的雅思写作考试成绩。朗阁海外考试研究中心的老师通过对过去题目的总结，发现可以这样写好文章的主题句以及支持句。
一、主题句
主题句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。
比如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.
主题句中提出的"certain rules"指的是什么？展开句中通过四个"Don''t argue --"逐一加以交代。从结构来看，这是一个比较典型的段落，它包括了主题句，推展句和结论句（即本段中的末句）。
1. 主题句的位置
主题句通常放在段落的开端，其特点是开门见山地摆出问题，然后加以详细说明。其作用是使文章的结构更清晰，更具说服力，便于读者*地把握主题和想象全段的内容。主题句可以放在段中起到承上启下的作用，或放在段尾起概括全段的作用。但初学者比较难于掌握，因而可尽量采用将主题句放在段落开头的写作手法。
比如：
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.
这段文字所讲的主要是能源问题。句就概括性地提出“我们目前的生活很大程度上依赖于能源”。而随后出现的三句都是具体事例，对句进行说明和论述。我们可以断定句为该段的主题句。
2. 如何写好主题句中的关键词
段落的主题句对主题的限定主要是通过句中的关键词来表现的。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法。准确地把握关键词是清楚地表达段落主题、写好段落主题句的重要前提之一。在上面的例1中主题句的关键词是：depends very much on energy。
主题句不能太抽象，要有具体内容。比如television is very useful就不能作为主题句。因为太抽象，没有说出具体内容。所以关键词要尽量写的具体一些。如果改为Television presents a vivid world in front of us或者Television plays an educational role in our daily life就比较好。
再比如She tries to improve her looks. 这个主题句的关键词就写得不好。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。如果改为She tries many ways to improve her looks. 或者There are some reasons why she tries to improve her looks就比较具体，进而支持句也比较容易展开了。
3. 如何写好主题句的中心思想：
主题句由两部分组成，即主题(topic)和中心思想(controlling idea)。中心思想的作用是导向(control)和制约(limit)。我们前面谈到的关键词就反映了中心思想。所谓导向就是规定段落的发展脉络，所谓制约就是限制主题的覆盖范围，两者不可分割。没有导向，内容就会离题或偏题；没有制约，内容就可能超出一个段落所能容纳的范围：比如The computer technology has no doubt at all replaced most of function, that is, we can read books from the academic database; newspapers and publishes from almost everywhere, this means that we do not need many hard copies and environment friendly for the earth. 本段的主题句对支持句起到了导向作用。
再比如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 the time 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，是短文写作成功的关键之一。
二、扩展句
1. 主要扩展句
主要扩展句(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*，句与句之间的关系是相互独立又是互相连接的。例如（主题句）There are several factors affecting climate. （扩展句1）One factor is the mount of sunlight received. （扩展句2）Altitude, or the height above sea level, also determines climate. （扩展句3）The oceans have an effect on the climate of adjacent land. （扩展句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。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. 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的句子。它从属于某一个或某几个推展句。
例如（主题句)）I don''t teach because teaching is easy for me. （主要推展句1）Teaching is the most difficult of 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
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