段落是由若干个彼此关联的句子构成的，通常用以表达一个中心思想或者从一个角度对文章的主题进行阐述。因此，段落不能是一组句子的任意堆砌，而是符合一定的模式，具有某些基本特征，遵循一定段落发展方法的。 当然，段落并没有固定的模式。在实践中，我们常常碰到各种各样的段落，有些仅仅充当导言或结束语，有些仅仅起过渡词作用，这种段落便不可能也不必要包含完整的思想内容。要写出一篇好文章，我们就必须了解一些结构完整的段落的写法，掌握一点组句成段的基本知识。 结构完整的段落大多由三部分组成： 一、主题句
主题句是概括段落中心思想，反映作者写作意图的一个概括性的句子，是段落的核心所在。所以，写好主题句是写好一个段落的前提。
1.段落主题句的位置及其作用
主题句通常由主题和作者的观点两部分组成，作为段落传递信息的出发点，简明扼要地告诉读者段落的线索和范围。但是，主题句的位置并不是固定不变的，它可以位于段首，也可以位于段中、段尾。主题句位于段首的情况比较多见。段首主题句开门见山，直接点明段落主题，有助于确定段落的主线，使各个发展句围绕段落主题自然展开；段尾主题句具有结论作用，是对前面句子的归纳总结。
请看下面各例：
Public opinions on what it takes to succeed in ones studies or career vary widely. Given approximately equal circumstances， some claim the success factor is largely a matter of luck—being in the right place and the right time. Others speak of utter devotion to work， combined with a degree of ruthlessness. Still others maintain that it is undoubtedly a matter of how much intelligence you have or simply how much education your mother had. In a word， people have different ideas on it.
本段的主题句Public opinions on what it takes to succeed in ones studies or career vary widely.放在段落的开头，开门见山地点明主题，使读者很容易抓住本段的写作主线：对学业或事业成功所持的各种不同的观点和看法。
The purpose of this book is to provide you with practice in spelling English. It is not a book that treats the complex rules of English orthography. Neither does it teach you how to use a dictionary. Its main objective is to introduce regular spelling patterns： that is， to present a regular system for spelling a large group of words. By learning these patterns， and by practicing them， you will master thousands of words.
Only the idle will be tired.
只有闲人才会感到疲倦。本段的主题句The purpose of this book is to provide you with practice in spelling English也是放在段落的开头，直截了当地告诉读者作者的写作意图，使读者对本段的主题思想有了明确的认识。
Give students a chance to grow. Do you mold them from one of a thousand patterns？ Let them seek knowledge， but do not find it for them. Let them learn patience； do not force it on them. Let them take their own time to grow； do not set rigid time schedules. Most of all， do not push them against a stone wall， crushing them with knowledge gained from the experience of others. Experience cannot be taught； it must come slowly through personal search.
在这个段落中，主题句先不出现，而是先举出许多具体的experiences，在段落结尾时才予以归纳，得出中心结论：Experience cannot be taught； it must come slowly through personal search.这样的段落结构一开始容易使读者产生悬念，从而收到特殊的表达效果。
Most of us may have such experiences： when you go to some place far away from the city you live in and think you know nobody there， you are surprised to find that you should run into one of your old classmates in the street. Perhaps both of you would cry out，What a small world！
在这个段落里，作者先给出了一个假设性的事例，吸引读者的注意力，然后在段落结束时总结出What a small world！（世界真小啊）作为段落主题句。这样的段落有感染力，使读者有兴趣读下去。
但是，主题句有时也可能出现在段落中间，或者根本就没有明显的段落主题句。在下面的例子中主题句是在段落中间出现的。这种情况比较少见。
Californians and New Englanders are both American. They speak the same language and abide by the same federal laws. But they are very different in their ways of life. Mobility—both physical and psychological—has made a great impression on the culture of Californians； lack of mobility is the mark of the customs and morality of New Englanders.
有时为了特别强调主题句的内容，也可以使主题句既出现在段首，又出现在段末。如：
Good manners are important in all countries， but ways of expressing good manners are different from country to country. Americans eat with knives and forks； Japanese eat with chopsticks. Americans sayHiwhen they meet； Japanese bow. Many American men open doors for women； Japanese men do not. On the surface， it appears that good manners in America are not good manners in Japan， and in a way this is true. But in any country， the only manners that are important are those involving one persons behavior toward another person. In all countries it is good manners to behave considerately toward others and bad manners not to. It is only the way of behaving politely that differs from country to country.
而下面这段话则没有明显的主题句。
An independent trucker working full time can earn close to $20，000 gross compared with the $30，000 or more that a union trucker makes. A union trucker works a 10hour shift and thats it， whereas the independent trucker is always pushing himself. And even though there are strict laws prohibiting a trucker from driving more than ten hours a day， with four hours on and four hours off， no independent trucker adheres to that—because if he did， hed never get the load delivered on time. With the economy as shaky as it is， the competition for loads is fierce and， at times， vicious. There are only so many loads， and truckers will bid on them， the lowest bid getting the load.
这段话的主题句应该是An independent trucker has a difficult time making a living.但由于暗含的意思很清楚，所以被省去。应该注意的是，在没有主题句的段落中，叙述应该十分清楚，暗含的主题必须显而易见。
There are two tragedies in life. One is to lose your hearts desire， the other is to gain it.
失去心中所欲的和得到心中所欲的，是人生的两大悲剧。特别提请注意的是，在段落中，虽然主题句可以放在不同的位置，但最常见的还是放在段首，这并不是说主题句一定就是该段的第一句话，而往往是在主题句之前还有一个承上启下的过渡句。
2.段落主题句的写作方法
主题句的作用是要告诉读者该段的主题思想，该段将围绕这个主题思想逐步展开、定义、论述、分类、解释和举例说明等，所以，段落主题句中必须包含一个主导思想，这是一个等待发展的思想，也是段落的主旨所在。同时，段落主题句不能写得太笼统，也不能涉及面太窄。一个段落的容量是有限的，如果主题句限定范围太宽，其内容就无法在一个段落中阐述清楚；如果限定范围太窄又不利于段落的发展。段落主题句所限定的内容必须符合段落写作的目的，有助于段落的铺开和抒发。例如：
Television is very important.这个句子太泛，没有表达一个清楚的主题思想，做主题句不大合适。可以改为：Television plays an educational role in our daily life.
Big cities have traffic problems.这个句子如果用做主题句也没有表达出明确的主题思想，范围限得过宽，抓不住段落主线，不好下笔扩展段落。可改为：Traffic problems in big cities are serious， which can be found in several ways.
He encountered an old friend yesterday.这是一个描写细节的句子。由于涉及面太窄而不利于展开讨论，不适合做主题句。
I go to college to make friends.如果作为段落的主题句就显得范围太窄了，作者发挥的余地太小，段落不容易写好。可改为：Going to college， I can learn more and make new friends. 二、扩展句
扩展句是用来支持或说明主题句的，是段落主题句的延伸和发展，起着辅助主题句、推展段落中心的作用，对主题句表达的中心思想或举例说明、或细致描写、或详细解释阐述和论证，使读者能够清晰地理解和信服地接受作者所表达的意思。
扩展句一般要具备这样的特点：（1）清晰、详实，有较强的说服力，能清楚地表达思想；（2）条理分明，脉络清晰。上一句要为下一句铺平道路，下一句是上一句的自然延伸，一步一步地论述或叙述主题。
一个段落通常有若干个扩展句。这些扩展句可以处于同一个层次，共同来为主题句服务；也可以在一级扩展句之下用二级扩展句来支持一级扩展句，从而构成扩展句之间的层次关系。但在段落中如果有多个不同层次的扩展句时，必须要合理地安排它们，既要作到简洁清晰、意义连贯、合乎逻辑，又要作到层次分明并紧紧地围绕主题句，为主题句服务；同时还要注意一级扩展句、二级扩展句等和主题句之间的关系。一级扩展句为主题句服务，二级扩展句支持一级扩展句来说明或论证主题句。
Strong in action， gentle in method.——行动要坚强，方式要温和。请看下面的例子：
（1）Money spent on advertising is money spent as well as any I know of.（2）It serves directly to assist a rapid distribution of goods at reasonable prices thereby establishing a firm home market and so making it possible to provide for export at competitive prices.（3）By drawing attention to new ideas， it helps enormously to raise standards of living.（4）By helping to increase demand， it ensures an increased need for labor， and is therefore an effective way to fight unemployment.（5）It lowers the costs of many services： without advertisements your daily newspapers would cost four times as much， the price of your television license would need to be doubled， and travel by bus or tube would cost 20 percent more.
这个段落中的句（1）是段落主题句，提出观点：花钱做广告是我所知道的最好的花钱方式。句（2）、（3）、（4）和（5）都是扩展句，处在同一层次上，进一步阐述和说明主题句，即通过列举广告给社会带来的好处来说明为什么花钱做广告是最好的花钱方式。
（1）The computer finds many applications in all fields. （2）Computers are now doing almost everything from diagnosing hospital patients to running nuclear power stations. （3）In any field where information is collected， processed， stored， and retrieved， computers are being used more and more. （4）Computers are also used as an aid to design planning. （5）For example， they can be used to predict faults in design as well as the cost of a design if one is planning to build a road.
在这个段落中，句（1）是主题句，提出计算机在所有领域都有应用的观点；句（2）、（3）和（4）是一级扩展句，都支持和说明句（1），具体说明计算机都在哪些领域应用，而句（5）是二级扩展句，用来补充说明句（4），通过举例来说明计算机辅助设计的用途。
（1）In rock music there is a distinct and lmost overwhelming beat. （2）No single beat is characteristic of the music today.（3）But each song has an easily recognizable rhythm. （4）As you listen to a song， your foot usually starts to pick up the beat. （5）Before long， your entire body seems to be moving with it.（6）Your head pounds with the beat， and there is no room for thought.（7）Only the surge of the music is important. （8）In its own way， rock music is as dominant as the rock Gibraltar. Its message is an overgrowing emotional one.
在这一段中，句（1）是段落主题句，点出本段主题思想，要说明摇滚乐最突出的特点是其节奏。句（2）、（3）是一级扩展句，对主题句进一步补充说明；而句（4）、（5）、（6）是二级扩展句，从属于句（2），对句（2）起着补充、解释和说明的作用，更形象、生动地证明了主题句所表达的思想。这里，句（7）是结尾句，它的功用在后面的章节里会做详尽的讲解。
He who mistrusts most should be trusted least.——最不信任别人的人最不应该得到信任。 三、结尾句
结尾句位于段落的末尾，在段落中也起着非常重要的作用，用以对全段内容进行总结、归纳或提出结论性的观点。结尾句常和主题句相呼应，以不同的形式再现主题，并与扩展句相关联。但是并非任何段落都必须有结尾句才算完整。有的段落，尤其是一些较短的段落中只有主题句没有结尾句。
请看下面各段落中结尾句的写法：
Scientists believe that there has been life of some sort on the earth for about twelve hundred million years. Such a large figure is difficult to grasp， so let us scale it down and reckon it as one hundred years. We then find that man has been on the earth for about a month， and civilized man for only seven or eight hours. So you see mankind is only at the beginning of its civilized life， and we must not expect too much.
本段的结尾句是对全段的总结归纳。它进一步强调了段落的中心思想，以引起读者的注意和重视。
A contract is an agreement which is enforceable by law. It can be long or short， formal or informal， simple or complicated， and verbal or written. Without a contract or agreement to bind the contracting parties， any international business or transaction would be impossible. Long age， people exchanged promises in making bargains and binding the conduct of others. This exchange of promises came to be known asagreementand became more and more important in the fields of business. A promise or agreement is reached as a result of the process of offer and acceptance. When an agreement is reached， a contract is formed. Once a contract is officially signed by the concerned parties， it creates legal obligations in the sense of law.
本段的最后一句是结尾句，第一句是主题句。结尾句所表达的意思跟主题句一致，但句子结构和用词都不一样。结尾句是主题句的再现，起着前后呼应的作用，段落中心思想更加突出明显。
The alarming problem is the increase in violent incidents owing to love on campus. When failing to win desirable affection from their lovers， some girl students tend to commit suicide while boys turn to knife and poison for revenge. Besides， it is a painful fact that boys obtain money needed for their love in an improper way. To please their girlfriends， boy students always dress smartly， shop generously and eat out frequently； but when finding no other quicker way to make the money needed than by stealing or cheating， they are on the way to imprisonment. Another sensitive problem is the growing pregnancy rate among girl students. The young students are subjected to blame for being too young to put themselves under sensible control， but school authorities cannot escape their share of responsibility.
本段的结尾句提出了结论性观点，既呼应了主题句内容，又与扩展句紧密关联，并且承上启下地引出后面段落的主题。
Eskimos are expert at fishing and hunting. Eskimos know the habits of each animal living in the northern lands. They know that a seal swimming under the ice will keep a breathing hole open by its warm breath， so they will wait beside the hole and kill it. Eskimos fish in the rivers， and when the sea is frozen， they cut holes in the ice and lower fishing lines into the water.
本段没有结尾句。
Idleness is the root of all evils.——懒惰是万恶之源
