主题句出现在段落首或尾完全由写作需要而定。一般地说来，写这样100多词的小短文把主题句放在段首更有利于考生扣准中心思想展开论述。 3.怎样写好主题句 主题句在段落中有着举足轻重的作用。因此写好主题句是写好段落的关键。在构思主题句时要注意以下三个方面： (1)主题句首先应是一个完整的句子，任何词组或修饰成分都不能作主题句。如：more burdens,就不是一个完整的句子。reading is thought to be a kind of conversation between the reader and the text，则是一个比较好的主题句。 (2)主题句不应太笼统概括。如：william shakespeare is great这句话很笼统，对段落如何展开没有指导和限定作用，因而不能作主题句。如改为：william shakespeare wrote several historic plays,则下文就能围绕莎氏的历史剧展开论述了。 (3)主题句不能太具体。如：the dictionary is small，句子如果太琐碎具体就失去进一步展开的意义了。 (4)各段的主题句应相互照应。在以 no smoking为题的作文中，各段的主题句分别是： smoking is harmful. smoking does not only harm the smokers but also people around them. therefore,smoking is a bad habit. 第二段的主题句用not only，but also连接词语沟通了上下两段的内容。第三段的therefore又起了承上启下的作用，使全篇融为一体。 (二)段落的展开 展开段落的方法有很多种，在这里我们仅介绍几种常用方法。 1.依据归纳法或演绎法进行论述 依据归纳法展开段落是指在段落中先引用具体事实或因由进行阐述或论说，进而得出结论。演绎法则是由一般推出特殊情况的结论。 下面这一段落是用归纳法展开的段落。最后一句是结论，也是主题句。 and that is exactly what reading a book should be：a conversation between you and the auther.presumably,he knows more about the subject than you do.naturally,you will have the proper humility as you approach him.but don′t let anybody tell you that a reader is supposed to be solely on the receiving end.understanding is a two？way operation.learning doesn′t consist in being an empty receptacle.the learner has to question himself and the teacher.he even has to argue with the teacher,once he understands what the teacher is saying.and marking a book is literally an expression of your differences,or agreements of opinion,with the author. 2.依据重要性展开段落 依据思想或事实或理由的重要性的先后次序进行论述，可以从次要至重要，也可以从重要至次重要。下面段落是以次重要至重要的方法进行论述的。作者首先指出(医生)对病人撒谎不仅对医生本人不好，也会伤害同事，进而会有损于整个医疗事业。 lies also do harm to those who tell them:harm to their integrity and,in the long run,to their credibility.lies hurt their colleagues as well.the suspicion of deceit undercuts the work of the many doctors who are scrupulously honest with their patiens.it contributes to the spiral of lawsuits and of "defen sive medicine",and thus it injures,in turn,the entire medical profession. 3.依据比较和对比法进行论述 一般地说，比较是指对事物的相同方面进行比较；对比是指对比事物的差异或不同方面。下面两段就采用了对比方法。这两段将口头英语与书面英语的不同方面作了比较，两段的观点都一一对应，比如：oral english is imformal while written english is comparatively formal就是一对观点的对比。另外，两段之间用unlike oral englsh承接，既让段落过渡自然，又使对比强烈。