在應試型（雅思或新托福等）議論文寫作中，通常遵循introduction- body-conclusion（引言段-主體段-結論段）的“三步曲”。Body（主體）段落提供了論證觀點的理由，是整個文章的主體，在評分中占有很大的比重。例如一篇滿分的雅思作文（9分）必須“presents a fully developed position in answer to the question with relevant, fully extended and well supported ideas”。這里的“fully extended / well supported”，必須通過主體段來體現。新托福的滿分作文（5分）也要求“is well organized and well developed, using clearly appropriate explanations, exemplifications, and/or details”，同樣必須通過主體段落發展來實現。

　　一般來說，一篇議論文必須包含至少兩個主體段。每個主體段都必須有明確的主題句“topic sentence”和若干支持句“supporting sentences”。他們共同組成文章的理由段，對全文的論點提供理由支持。建議初學者在練習議論文寫作時，必須遵循理由段發展的幾個簡單原則，這對迅速完成理由段、構建連貫和理由充分的議論文大有幫助。這幾個簡單原則包括：

　　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.

　　（每個主體段都必須有一個明確的主題句）

　　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.

　　（支持句必須圍繞主題句展開）

　　根據第2條的原則，我們來看看下面這個主體段：

　　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.

　　這段話的topic sentence很明顯-“Hobbies are important for many reasons”，之后的支持句從三方面闡述hobby的重要性（first, second, third），但段中這句話"A person can also meet other people by going the school"與hobby重要的原因沒有關系，因此削弱了整個段落的連貫性與統一性，應該被去掉。

　　3.    Add details. To write a more fully developed paragraph, you need to add details to your supporting ideas. Your details can be facts, examples, personal experiences, or descriptions.

　　（需要用多種方式提供細節展開段落）

　　根據第3條的原則，我們來看下面這個段落：

　　The Smithsonian Institution is worth visiting for a number of reasons. The Smithsonian Institution comprises various museums that offer something for everyone. These museums include the National Museum of History and Technology, the National Aeronautics and Space Museum, the National Collection of Fine Arts, the National Museum of Natural History, and several others. A person can do more than just look at the exhibits. For example, in the insect zoo at the National Museum of Natural History, anyone who so desires can handle some of the exhibits. The museums provide unforgettable experiences. In climbing through the Skylab exhibit at the National Aeronautics and Space Museum, I was able to imagine what it would be like to be an astronaut in space. Movies shown at regular intervals aid in building an appreciation of our world. In the National Aeronautics and Space Museum, there is a theater that has a large screen. When the movie is shown, it gives the viewer the feeling that he or she is in the movie itself, either floating above the Earth in a hot-air balloon or hang gliding over cliffs.

　　這是一個成功的主題段，請注意這段話段落細節展開的幾種形式-提供事實、舉例、個人經歷和描述。我們把這個段落拆分后進行分析：

　　Topic sentence（主題句）

　　The Smithsonian Institution is worth visiting for a number of reasons.

　　Supporting idea 1（支持理由1）

　　The Smithsonian Institution comprises various museums that offer something for everyone.

　　Details – facts（細節展開-提供事實）

　　These museums include the National Museum of History and Technology, the National Aeronautics and Space Museum, the National Collection of Fine Arts, the National Museum of Natural History, and several others.

　　Supporting idea 2（支持理由2）

　　A person can do more than just look at the exhibits.

　　Details – examples（細節展開-舉例）

　　For example, in the insect zoo at the National Museum of Natural History, anyone who so desires can handle some of the exhibits.

　　Supporting idea 3（支持理由3）

　　The museums provide unforgettable experiences.

　　Details – personal experience（細節展開-個人經歷）

　　In climbing through the Skylab exhibit at the National Aeronautics and Space Museum, I was able to imagine what it would be like to be an astronaut in space.

　　Supporting idea 4（支持理由4）

　　Movies shown at regular intervals aid in building an appreciation of our world.

　　Details – description（細節展開-描述）

　　In the National Aeronautics and Space Museum, there is a theater that has a large screen. When the movie is shown, it gives the viewer the feeling that he or she is in the movie itself, either floating above the Earth in a hot-air balloon or hang gliding over cliffs.

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