在應(yīng)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(jié)論段）的“三步曲”。Body（主體）段落提供了論證觀點(diǎn)的理由，是整個(gè)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(xiàn)。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(fā)展來實(shí)現(xiàn)。
一般來說，一篇議論文必須包含至少兩個(gè)主體段。每個(gè)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(diǎn)提供理由支持。初學(xué)者在練習(xí)議論文寫作時(shí)，必須遵循理由段發(fā)展的幾個(gè)簡單原則，這對迅速完成理由段、構(gòu)建連貫和理由充分的議論文大有幫助。這幾個(gè)簡單原則包括：
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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.
?。總€(gè)主體段都必須有一個(gè)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.
?。ㄖС志浔仨殗@主題句展開）
根據(jù)第2條的原則，我們來看看下面這個(gè)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
