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(AffirmativeSentence)
Example：Theneedforwildlifeprotectionisgreaternowthaneverbefore.
2)反诘句(RhetoricalSentence)
Example：Howdoyouthinkpeoplewillsolvetheproblemofwildlifeprotection
3)不完整句(Fragments)
Example：Andtheworkingman
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(Atthebeginning)
2)段末(Attheend)
用推理方法展开段落时，主题句往往位于句末。
3)段中(Inthemiddle)
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。
Exercise2-1
Directions：Readthefollowingparagraphsandidentifythetopicsentence.Ifitisimplied，summarizeityourself.
(1)
Butnomatterwhatitiscalled，allpolyesterhascertaingoodpoints.Itdoesnotwrinkleeasily.Itdriesquicklyafteritiswashed.Itholdsitsshape.Itisstrongandkeepsitscolorswell.
TopicSentence：____________________________________________
Position：________________
(2)
Blackisthecolourofmourning.Redsymbolizesdanger，violence，orbloodshed.Ifyouareafraid，youareyellow.NoneofthesesayingsistrueoutsidetheEnglishspeakingworld.InChinaandKorea，whiteisthecolorofmourning.InRussiaredstandsforbeautyandlife.InItalyandGermanyyouareyellowwithanger，notwithfear.
TopicSentence：_____________________________________________
Position：__________________
(3)
Ourchiefsourceoffreshwaterisrainfallthatcollectsinlakes，rivers，andreservoirs.Recently，however，wehavediscoveredanewsource，aquifers，whicharerockformationscontainingwater.Evenunderdeserts，vastaquifersmaybewaitingtobetapped.CompaniesdrillingforoilintheMiddleEasthavesometimesstruckaquifers，whichprovideunexpectedwatersuppliesforaridregions.
Position：_____________
(4)
Supposeyoureplayingagame.Youmakeasillymistakeandlose.DoyoubecomeangryOrcanyoulaughatyourselfandhopetodobetternexttimeSupposeyouareataspecialdinner.Youaccidentallyspillsomefood.WhykeepworryingabouthowclumsyyoulookWhynotlaughitoffandenjoyyourselfanywayIfyoucan，itsgoodsignyouvereallygrownup.
Position：____________
(5)
Youngmensometimesbringedelweissbackfortheirgirlfriends.Thegirlsthinkoftheflowersasaproofoftruelove.Mountainclimbers，guides，andhunterspickedelweisstoo.Theywearitontheirhats.Tothemitisasignofcourageanddaring.