什么是主题句？有什么特点？

　　什么是主题句？有什么特点？

　　主题句是表示全段中心思想的句子，是段落展开的基础，它表明作者的态度、观点、目的等.主题句一般简明扼要，具有概括、抽象的特点。主题句是对文章段落意义的表述，文章中其他句子都受主题句的制约。换言之.文章段落的其他句子是对主题思想加以阐述、说明或论证，是语义层层扩展、步步深人的核心。由于主题句的概括、抽象等特点，也称其为概述句。写作时，不可将某个细节或具体事实当做主题句来写。请看下段:

　　Machine scored test is called "objective" type. In an objective test, the students answerquestions by deciding on the best choice among a number of alternatives given. Another one isthe "completion" type. This kind of examination requires the students to add a word or phraseto a sentence. And the third type is the essay examination, in which the students are asked towrite a composition on a given topic.

　　这是一段没有主题句的段落，因为段中任何一句都不能概括全段内容，各句各自为政，均只是分别说明三种考试的特点。试比较下面经过修改的段落:
　　There are three kinds of examinations. One is the machine scored "objective" type. In anobjective test, the students answer questions by deciding on the best choice among a numberof alternatives given. Another is the "completion" type. This kind of examination requires thestudents to add a word or phrase to a sentence. And the third type is the essay examination, inwhich the students are asked to write a composition on a given topic. All these kinds of examinations are designed to reveal what a student may have learned in any particular course.

　　通过与范例1进行比较，我们可以看到，处于段首的"Examinations are of three kinds"显然是该段的主题句，因为这一句不仅具有概括全段的特点，而且全段的其他文字都是围绕着三种考试形式展开的。该段添加.七主题句后，使得段落的发展有了中心，读者读起来主题明确，论据充分。同时，为了进一步体现段落的完整性，实现首尾相呼应，该段在段末添加上了结论句"All these kinds of examinations are designed to reveal what a student may have learned inany particular course"，以达到对主题的进一步强调和突出.

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