所谓主题句(topic sentence, 也有人叫它中心句)，就是独立写作主体段中统领全文的那个句子，也就是表述在独立写作头脑风暴和谋篇布局时候想到的“主要理由”或者“论点”的那个句子。通常一篇独立写作有三个主题句。
例如在题目“Do you agree or disagree with the following statement? It is betterto work in large companies than in small ones.”，选择agree，三个主题句可以是：
The first reason is that a large company can provide more opportunities to develop one’s ability
Another reason for choosing a big company involves welfare.
A third reason is that one who appreciates the unique culture of abig enterprise will become a more responsible person to the society.
在托福写作中，三个主题句可以说是作文中最重要的一组句子，因为主题句对段落起到“提纲挈领”的作用，同时对整篇文章的结构清晰度和文章逻辑性也至关重要。
如何将这个理由“完整”“清晰”“出彩”的表达出来就是本文讨论的话题。
首先，每段主题句通常都位于段首，紧跟于“first, second, third”这些“信号词”之后。除此之外还需要注意以下几点。
丨简明扼要
主题句需要让读者看到之后能够快速、准确地把握本段的主要内容。这要求我们在写主题句的时候，一方面内容要简单，另一方面用语要凝练。
先来看一个反面例子：
“First and foremost, television, invented in the last century, withits wide availability and increasingly prosperous programs ,becomes one of the most powerful means of communication in history, and is more and more difficult to ignore”
这个主题句涵盖的内容太多，读完这个句子，读者根本不明白本段是要陈述电视节目蓬勃发展(increasingly prosperous program), 要强调电视是最强力的交流工具(the most powerful means of communication in history),还是要论证电视不可被忽视(difficult to ignore)，这就是一个典型的内容太复杂的主题句。
再看一个反例:
“First of all, following the new customs can show a sense of respect,which can allow a better and faster adaption inside the local population so that they make more friends.”
这个句子也有两个论点：表示尊重(show a sense of respect)，扩大社交(make more friends),同样也使读者不能抓住本段的主要内容。
如何避免这种错误，使主题句内容简单呢?很简单，首先做到每个主题句有且只有一个论点。论点就是指我们头脑风暴时候想到的那些“key words”，比如健康，安全，情感等等，每段写一个，不要把健康和安全放在同一个段落，也不要把情感和成功放在同一个段落。
例如上面第二个例子，我们只需要删去一个论点，就可以变得非常简洁：
“To start with, accepting cultures in the foreign country is an indispensable element that contributes to the expansion of social circle.”这是一个简洁的主题句，只有扩大社交(contributes to the expansion of social circle)这一个论点。
论点唯一还不一定能完全做到简洁，来看另一个反例：
“The first reason why letting children care for animals isnot the best way to teach them about responsibility is that it could negatively impact a child’s health.”
这句话虽然只有一个论点，可是用语太繁杂，让人头晕。主题句的语法不建议太复杂，建议大家把花式操作留到其他部分去秀，在主题句，只要写一些简单句，例如：“First, raising pets will exert a negative impact on a child’shealth.”就行了。
丨不是陈述事实
来看下面两个句子：
A: “First, some children might be infected by feeding animals.”
B: “First, raising pets could negatively impact a child’s health.”
哪一个是好的主题句呢?
答案是B。
因为A句的内容是一个“纯粹的事实”，而B句则是“抽象的概念”(exert negative impact)，换言之，A句可以作为B句的例子，但B句不能作为A句的例子。
陈述事实的句子是不能作为主题句的，事实是不言自明的。不言自明，也就不需要后面的文字来“论证”了。主题句需要写“a sentence that you could give examples for”，而不能写一个 “example”。
来练习判断下面几组句子中哪一个可以做主题句呢?
A: “Second, the academic performance of some children becomes poor after they are responsible for caring for a pet.”
B：“Second, caring for a pet could disrupt a child’s regular studies.”
A: “First, people could learn how to communicate with eachother through participating in community activities”
B: “First, participating in community activities is apractical and effective approach to enhance their social skills.”
A: “First, letting children take care of animal is a good suggestion for the reason that kids like animals.”
B: “To start with, raising pets fill friendship vacuums and satisfy people’s need to nurture"
(答案：三组都是B句较好)
丨使用高级词汇
很多同学要问，如果主题句中不建议使用复杂句型，又不能出现多层结构，那如何显示自己的语言功力呢?要知道我们展示语言能力的地方并非只有复杂的句型，丰富和精准的词汇使用，同样可以展示英语的专业程度。比如
“First, go to museums can teach people different kinds of knowledge”
→ “First, visiting museums provide people with an opportunity to comprehend a vast amount of knowledge”
想想看，如果原本句子中只会写“good”之处，替换为“advantageous, beneficial, effective, efficacious, favorable,invaluable, rewarding, unparalleled, unprecedented”等等词汇，效果是不是更好呢?如果多次出现“important”的地方，改写为“central, critical ,crucial, decisive, essential, pivotal, primary,principal, vital，a key to, an indispensable part, play a pivotal role, attach great importance to”会不会增加可读性呢?
在平时的阅读中，注意积累一些“高级词汇”，准确掌握词义，并且刻意练习使用这些词汇，逐渐就会取得不错的写作成绩。
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