段落是由若干個(gè)彼此關(guān)聯(lián)的句子構(gòu)成的，通常用以表達(dá)一個(gè)中心思想或者從一個(gè)角度對(duì)文章的主題進(jìn)行闡述。因此，段落不能是一組句子的任意堆砌，而是符合一定的模式，具有某些基本特征，遵循一定段落發(fā)展方法的。
當(dāng)然，段落并沒(méi)有固定的模式。在實(shí)踐中，我們常常碰到各種各樣的段落，有些僅僅充當(dāng)導(dǎo)言或結(jié)束語(yǔ)，有些僅僅起過(guò)渡詞作用，這種段落便不可能也不必要包含完整的思想內(nèi)容。要寫(xiě)出一篇好文章，我們就必須了解一些結(jié)構(gòu)完整的段落的寫(xiě)法，掌握一點(diǎn)組句成段的基本知識(shí)。
結(jié)構(gòu)完整的段落大多由三部分組成：
一、主題句
主題句是概括段落中心思想，反映作者寫(xiě)作意圖的一個(gè)概括性的句子，是段落的核心所在。所以，寫(xiě)好主題句是寫(xiě)好一個(gè)段落的前提。
1.段落主題句的位置及其作用
主題句通常由主題和作者的觀點(diǎn)兩部分組成，作為段落傳遞信息的出發(fā)點(diǎn)，簡(jiǎn)明扼要地告訴讀者段落的線索和范圍。但是，主題句的位置并不是固定不變的，它可以位于段首，也可以位于段中、段尾。主題句位于段首的情況比較多見(jiàn)。段首主題句開(kāi)門(mén)見(jiàn)山，直接點(diǎn)明段落主題，有助于確定段落的主線，使各個(gè)發(fā)展句圍繞段落主題自然展開(kāi)；段尾主題句具有結(jié)論作用，是對(duì)前面句子的歸納總結(jié)。
請(qǐng)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.
本段的主題句Public opinions on what it takes to succeed in ones studies or career vary widely.放在段落的開(kāi)頭，開(kāi)門(mén)見(jiàn)山地點(diǎn)明主題，使讀者很容易抓住本段的寫(xiě)作主線：對(duì)學(xué)業(yè)或事業(yè)成功所持的各種不同的觀點(diǎn)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.
只有閑人才會(huì)感到疲倦。本段的主題句The purpose of this book is to provide you with practice in spelling English也是放在段落的開(kāi)頭，直截了當(dāng)?shù)馗嬖V讀者作者的寫(xiě)作意圖，使讀者對(duì)本段的主題思想有了明確的認(rèn)識(shí)。
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.
在這個(gè)段落中，主題句先不出現(xiàn)，而是先舉出許多具體的experiences，在段落結(jié)尾時(shí)才予以歸納，得出中心結(jié)論：Experience cannot be taught； it must come slowly through personal search.這樣的段落結(jié)構(gòu)一開(kāi)始容易使讀者產(chǎn)生懸念，從而收到特殊的表達(dá)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！
在這個(gè)段落里，作者先給出了一個(gè)假設(shè)性的事例，吸引讀者的注意力，然后在段落結(jié)束時(shí)總結(jié)出What a small world！（世界真小?。┳鳛槎温渲黝}句。這樣的段落有感染力，使讀者有興趣讀下去。
但是，主題句有時(shí)也可能出現(xiàn)在段落中間，或者根本就沒(méi)有明顯的段落主題句。在下面的例子中主題句是在段落中間出現(xiàn)的。這種情況比較少見(jiàn)。
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.
有時(shí)為了特別強(qiáng)調(diào)主題句的內(nèi)容，也可以使主題句既出現(xiàn)在段首，又出現(xiàn)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.
而下面這段話則沒(méi)有明顯的主題句。
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.
這段話的主題句應(yīng)該是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。應(yīng)該注意的是，在沒(méi)有主題句的段落中，敘述應(yīng)該十分清楚，暗含的主題必須顯而易見(jiàn)。
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，是人生的兩大悲劇。特別提請(qǐng)注意的是，在段落中，雖然主題句可以放在不同的位置，但最常見(jiàn)的還是放在段首，這并不是說(shuō)主題句一定就是該段的第一句話，而往往是在主題句之前還有一個(gè)承上啟下的過(guò)渡句。
2.段落主題句的寫(xiě)作方法
主題句的作用是要告訴讀者該段的主題思想，該段將圍繞這個(gè)主題思想逐步展開(kāi)、定義、論述、分類、解釋和舉例說(shuō)明等，所以，段落主題句中必須包含一個(gè)主導(dǎo)思想，這是一個(gè)等待發(fā)展的思想，也是段落的主旨所在。同時(shí)，段落主題句不能寫(xiě)得太籠統(tǒng)，也不能涉及面太窄。一個(gè)段落的容量是有限的，如果主題句限定范圍太寬，其內(nèi)容就無(wú)法在一個(gè)段落中闡述清楚；如果限定范圍太窄又不利于段落的發(fā)展。段落主題句所限定的內(nèi)容必須符合段落寫(xiě)作的目的，有助于段落的鋪開(kāi)和抒發(fā)。例如：
Television is very important.這個(gè)句子太泛，沒(méi)有表達(dá)一個(gè)清楚的主題思想，做主題句不大合適?？梢愿臑椋篢elevision plays an educational role in our daily life.
Big cities have traffic problems.這個(gè)句子如果用做主題句也沒(méi)有表達(dá)出明確的主題思想，范圍限得過(guò)寬，抓不住段落主線，不好下筆擴(kuò)展段落?？筛臑椋篢raffic problems in big cities are serious， which can be found in several ways.
He encountered an old friend yesterday.這是一個(gè)描寫(xiě)細(xì)節(jié)的句子。由于涉及面太窄而不利于展開(kāi)討論，不適合做主題句。
I go to college to make friends.如果作為段落的主題句就顯得范圍太窄了，作者發(fā)揮的余地太小，段落不容易寫(xiě)好?？筛臑椋篏oing to college， I can learn more and make new friends.
二、擴(kuò)展句
擴(kuò)展句是用來(lái)支持或說(shuō)明主題句的，是段落主題句的延伸和發(fā)展，起著輔助主題句、推展段落中心的作用，對(duì)主題句表達(dá)的中心思想或舉例說(shuō)明、或細(xì)致描寫(xiě)、或詳細(xì)解釋闡述和論證，使讀者能夠清晰地理解和信服地接受作者所表達(dá)的意思。
擴(kuò)展句一般要具備這樣的特點(diǎn)：（1）清晰、詳實(shí)，有較強(qiáng)的說(shuō)服力，能清楚地表達(dá)思想；（2）條理分明，脈絡(luò)清晰。上一句要為下一句鋪平道路，下一句是上一句的自然延伸，一步一步地論述或敘述主題。
一個(gè)段落通常有若干個(gè)擴(kuò)展句。這些擴(kuò)展句可以處于同一個(gè)層次，共同來(lái)為主題句服務(wù)；也可以在一級(jí)擴(kuò)展句之下用二級(jí)擴(kuò)展句來(lái)支持一級(jí)擴(kuò)展句，從而構(gòu)成擴(kuò)展句之間的層次關(guān)系。但在段落中如果有多個(gè)不同層次的擴(kuò)展句時(shí)，必須要合理地安排它們，既要作到簡(jiǎn)潔清晰、意義連貫、合乎邏輯，又要作到層次分明并緊緊地圍繞主題句，為主題句服務(wù)；同時(shí)還要注意一級(jí)擴(kuò)展句、二級(jí)擴(kuò)展句等和主題句之間的關(guān)系。一級(jí)擴(kuò)展句為主題句服務(wù)，二級(jí)擴(kuò)展句支持一級(jí)擴(kuò)展句來(lái)說(shuō)明或論證主題句。
Strong in action， gentle in method.——行動(dòng)要堅(jiān)強(qiáng)，方式要溫和。請(qǐng)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.
這個(gè)段落中的句（1）是段落主題句，提出觀點(diǎn)：花錢(qián)做廣告是我所知道的的花錢(qián)方式。句（2）、（3）、（4）和（5）都是擴(kuò)展句，處在同一層次上，進(jìn)一步闡述和說(shuō)明主題句，即通過(guò)列舉廣告給社會(huì)帶來(lái)的好處來(lái)說(shuō)明為什么花錢(qián)做廣告是的花錢(qián)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.
在這個(gè)段落中，句（1）是主題句，提出計(jì)算機(jī)在所有領(lǐng)域都有應(yīng)用的觀點(diǎn)；句（2）、（3）和（4）是一級(jí)擴(kuò)展句，都支持和說(shuō)明句（1），具體說(shuō)明計(jì)算機(jī)都在哪些領(lǐng)域應(yīng)用，而句（5）是二級(jí)擴(kuò)展句，用來(lái)補(bǔ)充說(shuō)明句（4），通過(guò)舉例來(lái)說(shuō)明計(jì)算機(jī)輔助設(shè)計(jì)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.
在這一段中，句（1）是段落主題句，點(diǎn)出本段主題思想，要說(shuō)明搖滾樂(lè)最突出的特點(diǎn)是其節(jié)奏。句（2）、（3）是一級(jí)擴(kuò)展句，對(duì)主題句進(jìn)一步補(bǔ)充說(shuō)明；而句（4）、（5）、（6）是二級(jí)擴(kuò)展句，從屬于句（2），對(duì)句（2）起著補(bǔ)充、解釋和說(shuō)明的作用，更形象、生動(dòng)地證明了主題句所表達(dá)的思想。這里，句（7）是結(jié)尾句，它的功用在后面的章節(jié)里會(huì)做詳盡的講解。
He who mistrusts most should be trusted least.——最不信任別人的人最不應(yīng)該得到信任。
三、結(jié)尾句
結(jié)尾句位于段落的末尾，在段落中也起著非常重要的作用，用以對(duì)全段內(nèi)容進(jìn)行總結(jié)、歸納或提出結(jié)論性的觀點(diǎn)。結(jié)尾句常和主題句相呼應(yīng)，以不同的形式再現(xiàn)主題，并與擴(kuò)展句相關(guān)聯(lián)。但是并非任何段落都必須有結(jié)尾句才算完整。有的段落，尤其是一些較短的段落中只有主題句沒(méi)有結(jié)尾句。
請(qǐng)看下面各段落中結(jié)尾句的寫(xiě)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.
本段的結(jié)尾句是對(duì)全段的總結(jié)歸納。它進(jìn)一步強(qiáng)調(diào)了段落的中心思想，以引起讀者的注意和重視。
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.
本段的最后一句是結(jié)尾句，第一句是主題句。結(jié)尾句所表達(dá)的意思跟主題句一致，但句子結(jié)構(gòu)和用詞都不一樣。結(jié)尾句是主題句的再現(xiàn)，起著前后呼應(yīng)的作用，段落中心思想更加突出明顯。
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.
本段的結(jié)尾句提出了結(jié)論性觀點(diǎn)，既呼應(yīng)了主題句內(nèi)容，又與擴(kuò)展句緊密關(guān)聯(lián)，并且承上啟下地引出后面段落的主題。
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.
本段沒(méi)有結(jié)尾句。
Idleness is the root of all evils.——懶惰是萬(wàn)惡之源。