在托福写作中不难发现议论文通常遵循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。

　　每个主体段都必须有一个明确的主题句
　　例句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.

　　支持句必须围绕主题句展开
　　例句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.

　　新托福满分作文(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，一篇议论文写作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。