第二单元 主题句简介　　
　　2．1主题句
　　作者的首要任务是让读者知道所写段落要谈的是什么，这就是每段的主题句的作用。因此主题句应该阐明段落的主要思想，所有支持主题句的细节和描述都与这一主要思想有关。
　　2．2主题句的形式
　　主题句通常有以下三种形式：
　　1）肯定句（Affirmative Sentence）
　　Example： The need for wildlife protection is greater now than ever before．
　　2）反诘句（Rhetorical Sentence）
　　Example： How do you think people will solve the problem of wildlife protection？
　　3）不完整句（Fragments）
　　Example： And the workingman？
　　初学者最好使用肯定句作为主题句。
　　2．3主题句的位置
　　主题句出现的位置有以下四种情况：
　　1）段首（At the beginning）
　　主题句经常居于段首，以便读者浏览主题句就可掌握文章的概要。这个位置适用于写提供信息或解释观点的段落。
　　2）段末（At the end）
　　用推理方法展开段落时，主题句往往位于句末。
　　3）段中（In the middle）
　　有时为了使段落多样化，主题句也可以居于段中。
　　4）隐含（Implied）
　　有时候，尤其在写叙述性或描写性段落时，当所有的细节都围绕着一个显而易见的主题时可以不用主题句。
　　Exercise 2-1
　　Directions： Read the following paragraphs and identify the topic sentence． If it is implied， summarize it yourself．　　
　　（1）　　
　　But no matter what it is called， all polyester has certain good points． It does not wrinkle easily． It dries quickly after it is washed． It holds its shape． It is strong and keeps its colors well．
　　Topic Sentence：____________________________________________
　　Position：________________　　
　　（2）　　
　　Black is the colour of mourning． Red symbolizes danger，violence，or bloodshed． If you are afraid， you are yellow． None of these sayings is true outside the English speaking world． In China and Korea， white is the color of mourning． In Russia red stands for beauty and life． In Italy and Germany you are yellow with anger，not with fear．
　　Topic Sentence：_____________________________________________
　　Position：__________________　　
　　（3）　　
　　Our chief source of fresh water is rainfall that collects in lakes，rivers， and reservoirs． Recently， however， we have discovered anew source， aquifers， which are rock formations containing water．Even under deserts， vast aquifers may be waiting to be tapped．Companies drilling for oil in the Middle East have sometimes struck aquifers， which provide unexpected water supplies for arid regions．
　　Topic Sentence：____________________________________________
　　Position：_____________　　
　　（4）　　
　　Suppose you’re playing a game． You make a silly mistake and lose． Do you become angry？ Or can you laugh at yourself and hope to do better next time？ Suppose you are at a special dinner．You accidentally spill some food．Why keep worrying about how clumsy you look？Why not laugh it off and enjoy yourself anyway？If you can， it’s good sign you've really grown up．
　　Topic Sentence：____________________________________________
　　Position：____________　　
　　（5）　　
　　Young men sometimes bring edelweiss back for their girlfriends． The girls think of the flowers as a proof of true love．Mountain climbers， guides， and hunters pick edelweiss too．They wear it on their hats． To them it is a sign of courage and daring．
　　Topic Sentence：____________________________________________
　　Position：____________