在应试型（雅思或新托福等）议论文写作中，通常遵循introduction-body-conclusion（引言段—主体段—结论段）的“三步曲”。Body（主体）段落提供了论证观点的理由，是整个文章的主体，在评分中占有很大的比重。例如一篇满分的雅思作文（9分）必须“presents a fully developed position in answer to the question with relevant, fully extended and well supported ideas”。这里的“fully extended / well supported”，必须通过主体段来体现。新托福的满分作文（5分）也要求“is well organized and well developed, using clearly appropriate explanations, exemplifications, and/or details”，同样必须通过主体段落发展来实现。

　　一般来说，一篇议论文必须包含至少两个主体段。每个主体段都必须有明确的主题句“topic sentence”和若干支持句“supporting sentences”。他们共同组成文章的理由段，对全文的论点提供理由支持。建议初学者在练习议论文写作时，必须遵循理由段发展的几个简单原则，这对迅速完成理由段、构建连贯和理由充分的议论文大有帮助。这几个简单原则包括：

　　1. Write a topic sentence for each paragraph you plan to write. Each topic sentence should relate to your thesis statement and introduce what the paragraph will be about. If you find that the topics you want to discuss do not support the thesis statement you have written, revise your thesis statement or reconsider your topic sentences.

　　（每个主体段都必须有一个明确的主题句。）

　　2. Write ideas that support your topic sentences. The topic sentence for each paragraph tells the reader what the paragraph will be about. The ideas stated in the rest of the paragraph should all relate to the topic sentence.

　　（支持句必须围绕主题句展开。）

　　根据第2条的原则，我们来看看下面这个主体段：

　　Hobbies are important for many reasons. First, a hobby can be educational. For example, if the hobby is stamp collecting, the person can learn about the countries of the world and even some of their history. Second, engaging in a hobby can lead to meeting other people with the same interests. A person can also meet other people by going to the school. Third, a person's free time is being used in a positive way. The person has no time to be bored or get into mischief while engaged in the hobby. Finally, some hobbies can lead to a future job. A person who enjoys a hobby-related job is more satisfied with life.

　　这段话的topic sentence很明显——“Hobbies are important for many reasons”，之后的支持句从三方面阐述hobby的重要性（first / second / third），但段中这句话“A person can also meet other people by going the school”与hobby重要的原因没有关系，因此削弱了整个段落的连贯性与统一性，应该被去掉。