1.主旨句。主旨句揭示该段内容的中心，放在段落的开头。从修辞上看，主旨句宜用比较简洁的句子，使读者易于领会该段的主旨：下面这段话还未有主旨句，请同学们阅读之后从A、B、C、D中选一个做主旨句。
主旨句。The hibenating animal's temperature drops to justover zero centigrade.It breathes only once every five minutes.It does not feel any pain.You can beat it without causing itto wake up.So hibernating animals look as if they were dead.
A.As you know,hibernation is much deeper than a commonsleep.
B.Animals need hibernation.
C.Hibernation is a very deep sleep.
D.How can hibernating animals sleep all through the winter?
上面B、D两句不能说明这段话的中心，不能做主旨句。A、C两句都能说明这段话的中心，但是A句不如C句简洁明了，所以用C句做这段话的主旨句最恰当。
2.扩展句。扩展句的内容必须围绕主旨句，支持主旨句。偏离主旨句的扩展句等于画蛇添足，应该删去。从修辞上看，几个扩展句如果能分别用简单句，并列句和复合句，则整个段落会显得生动一些，而不至于那么单调、呆板。下面这段话里，有一个扩展句是偏离主旨的，请同学们把它找出来。
It is easy to do this experiment.First,you get a giass ofwater and a pencil.Then you put the pencil into the water.Atthis time,you will see the pencil appears broken.Perhaps youhave once found this thing before.The expenment shows thatlight waves change speed when they go from air into water.
在这段话里,First..,Then...和At this time...这三个扩展句都是说明“这个实验很容易做”这个主旨的。但是Perhaps...这个句子偏离了这个主旨，它跟这个实验的做法无关，应该删去。
扩展句可以按照时间、方位、因果、对比、类别等关系来组织。例如上面这段话里的扩展句是以时间为序来安排的。“组段”的开头，说竹有很多用途那段话，是以方位(茎、根、髓)为序来安排扩展句的。
3.结尾句。结尾句是对主旨句的呼应，它通常导出扩展句所陈述的结果。下面这段话还没有结尾句，请同学们阅读之后，从A、B、C、D中选一句做它的结尾句。
How can we stop pollution?The most important thing is topass certain laws to prevent factories from sending out noise,poisonous gases and dirty water.Besides,we should protectforests and plant more trees.Meanwhile,every one of us mustpay close attention to public health and keep oursurroundings clean.
A.In spite of all these measures,we still cannot defeatpollution.
B.So pollution is a serious problem that needs all thesociety to care about.
C.However,pollution will still exist and we must not letdown our guard against it
D.In this way,we shall win the battle against pollution.
显然，D句与主旨句呼应最密切，它导出了立法、保护森林、植树、注意公共卫生和搞好环境清洁等措施的结果。所以，应该选D句做本段的结尾句。
四、谋篇
一篇文章应有比较完整的内容和结构。它通常包含引言段、扩展段和结论段。这和我们汉语作文的起、承、转、合是基本一致的。下面请读一篇论述时间的价值的文章。
The Value of Time
A proverb says:"Time is money."But in my opinion,time is even more precious than money.Why?Because when money is spent,we can earn(赚)it back.However,when time is gone,it willnever return.This is the reason why we must value time.
It goes without saying that the time we can use is limited.Therefore,even an hour is extremely precious.We shouid makefull use of our time to do useful things.As students we mustnot relax(放松)our efforts to engage in our studies so asto serve society and our nation in the future.
But it is a pity that there were a lot of people who do notknow the importance of time.They spend their precious timesmoking,drinding and chatting.They do not realize thatwasting time is equal to wasting a part of their valuablelife.
In a word,we should form the good habit of saving time.Donot put off what can be done today until tomorrow.
这篇文章有四段。第一段是引言段，提出时间比金钱更宝贵，我们必须珍惜时间这个论题。第二段是扩展段，说明时间是有限的，作为学生更要充分利用它来学习，以利将来为国家服务。第三段也是扩展段，指出有些人还没有认识时间的价值。第四段是结论段，作出我们必须养成节约时间习惯的诊断。这四段话刚好是起——承——转——合的结构。
从这篇文章还可以看到，英语作文的起承转合会用上一些特定的词语。下面分别作简单的介绍。
1.常用在引言段开头(起)的词语：
Generaily speaking,...一般地说，……
A proverb says,"..."有句谚语说，“……”
Many people often ask this question,"..."许多人常常问这个司题，“……?”
First of all...首先……/At present...当今……
2.常用在第一个扩展段开头(承)的词语：
To take...for an example，……以……为例
Now that we know that...既然我们知道……
What is more important is that...更为重要的是……
That can be expressed as follows:...这可以分述如下：……
in other words，换句话说
second，第二，
3.常用在第二扩展段开头(转)的词语：
But it is a pity that...但是很可惜，……
But the problem is not so slmple.但是问题并非如此简单。
On the other hand,...另一方面，……
On the contrary,...相反……
However,...然而……
4.常用在结论段开头(合)的词语：
In a word,...总而言之，……
On account of this,we can find that...由此我们可以知道……
In conclusion，结论是……
In short,...简言之，……
As has been noted...如前所述……
Therefore,...因此，……
At last,...最后，……
下面有一篇文章谈论“我们为什么要参加体育运动”。它的引言段、扩展段和结论段的次序是混乱的，请同学们按起承转合的顺序把它们排正，并注意段落开头表示起承转合的词语。
Why should we take part in sports and games?
①Therefore,every one of us,young or old,man or woman,should take an active part in sports and games.
②Besides,sports and games are also good for character-training.Unselfishness,courage,discipline,love of one'scountry and so on,are virtues that a citizen should have.Students learn a lot about these from books.But in sportsand games they learn to play fair,cooperate with other teammembers,go all out to win the match and think of honor of thegroup.All this will help them to become better eitizens.
③Everybody knows that sports and games build our bodies,prevent us from too fat and keep us healthy.They also give usvaluable practice in helping the eyes,brain and muscles worktogether.Such kind of practice is especially useful for thosewho work with their brains most of the time.
④It goes without saying that sports and games are not onlyamusements.They are of great value both to one's body andmind.
上文四个段落的正确排列次序是④—③—②—①。还要指出的是，这篇文章的两个扩展段③和②，都是说明体育运动的价值，起“承”的作用，所以这篇文章没有“转”这个环节。
最后尚需强调的是在命题作文中有很多学生没有表达主要思想，很多学生不会运用细节来扩充主题句。这是因为在平时训练中多为“词—句—段”式。缺乏衔接性、逻辑性等，致使他们的写作水平停留在词汇、语法、句型的运用上。须知，内容是文章的灵魂，是前提，也是写作的目的，应加足够注意。同时要从语篇水平上考虑谋篇布景，内容表达、语法修辞等诸方面的训练，这应是写作教学的重点之一。
下一篇：浅谈语境与词语的异常搭配