无论是中文还是英语抑或其他语言，我们知道写文章势必要有一个中心思想，而每个段落只能有一个主题（central idea），它用一个句子加以表达，就是主题句。几个主题句的精彩写作不仅会让文章条理分明，更能够很好的诠释文章主题思想，可谓是文章的架构灵魂，所以，写好主题句对一篇文章的好坏有着至关重要的作用。

首先，充分了解主题句。

定义：主题句为一个完整的句子，用以概括、叙述和说明该段的主题。

位置：主题句一般位于段首，也可置于段尾或段中，考生最好放在段首，这样较易掌握和构思。

例如：

位于段首：在英文中，大部分段落的第一句便是主题句。开门见山地提出问题，后面的扩展句围绕主题句加以说明、支持、补充和解释。

An important part of police strategy, rapid police response is seen by police officers and the public alike as offering tremendous benefits. The more obvious ones are the abilities of police to apply first aid life saving techniques quickly, and the greater likelihood of arresting people who may have participated in a crime. It aids in identifying those who witnessed an emergency or crime as well as in collecting evidence. The overall reputation of a police department too is enhanced if rapid response is consistent and this in itself promotes the prevention of crime. Needless to say rapid response offers the public some degree of satisfaction in its police force.

位于段尾：主题句位于段尾便于总结全段的内容，给读者以深刻的印象，是写作中的一种演绎方法。

At present rates of demand, the world has enough oil to last for more than 40 years, enough gas for more than 60 years and enough coal for more than 230 years. Naturally, demand will increase; but so will reserves as companies explore more widely and costs fall. Since 1970 viable reserves of oil have almost doubled while those of gas have leapt three-fold. One distant day a crunch will come, but as it approaches fossil-fuel prices will rise, making alternative forms of energy, perhaps including nuclear power, competitive. That is no reason to spend on nuclear now.

位于段中：位于段中的主题句起承上启下的作用，这类主题句多起转折作用，一般由“but, however, yet, anyhow, nevertheless”等词连接，用于引起下文。

What we teach ourselves sometimes, indeed, is more useful than what we learn from others. Some great men had little or no schooling. But these great men probably studied harder by themselves than most boys do in school. The greatest minds do not necessarily of those who have never been able to distinguish themselves at school, have been very successful in life later. It has been said that Wellington and Napoleon were both dull boys at school, and so were Newton and Albert Einstein.

无主题句：有的段落中无主题句，段落的主题思想通过文章内容来提示，这种方法能促使读者对文章内容进行思考和分析，但考生一般不宜在考试中采用这种方法：

Money may be deposited or withdrawn from branches of the credit unions and banks during business hours from Monday to Friday (but not on public holidays) or 24 hours a day from the many on-campus automatic teller machines. Business hours for financial institutions vary, but credit unions are usually open from 9 am to 5 pm weekdays, and, generally, banks are open from 9.30 am to 4 pm (5 pm on Fridays). Some services are available on Saturday mornings in selected areas. While prices often compare favorably with prices overseas, because Australia is a large and exciting country it is very easy to overspend, especially if on a tight budget.

其次，如何写好主题句。

写好主题句的方法：

1）主题句要概括一定内容，不要空泛，否则扩展句将难以说明和支持它，例如：

空泛：English language is very important.

概括：English language is very important in our daily life.

空泛：The Olympic Games are exciting.

概括：In the Olympic Games the football teams from many countries compete intensely.

2）尽量使用简单句或简洁明了的句子，例如：：

简洁：Collecting stamps is her hobby.

复杂: She likes collecting stamps which is her hobby.

简洁: I enjoyed watching Gone with the wind very much.

复杂：Gone with the wind was a good film which I enjoyed watching very much.

3）主题句应该做到句子完整和表达的主题思想完整，：例如：

不完整：How to write a composition.

完整：How to write a composition is not an easy thing to talk about.

不完整：If the weather had been fine.

完整：If it had been fine, we would have had a good time.

4）关键词是直接表达主题的词汇，它决定段落的内容和展开的方法，引导整个段落的发展，例如：

There are several ways to boil the water.

The task can be finished in three steps.

There is a new method to reduce the cost.