一、段落基本句型构造
1.说明危害段
说明危害段一般出现在对策方法类作文的第二段或是利弊分析类作文的第二段，其基本的句型构造是：先用一句话总述问题的危害或弊端，这是段落的主题句；然后用3-4句话列举2-3点具体的危害或弊端，以支持主题句；最后再用一句话对这些危害或弊端作出总结，引出下面对解决方法或观点的阐述（本句也可能放在下一段的开头）。
例
directions : for this part , you are allowed 30 minutes to write a composition on the topic reduce waste on campus . you should write at least 120 words following the outline given below in chinese :
1.目前有些校园内浪费现象严重。
2.浪费的危害。
3.倡导从我做起，杜绝浪费。
第二段：说明危害段
(1)as everyone sees , this phenomenon prouces harmful effects in many aspects . (2) above all , the waste of those precious resources , such as water and electricity , will bring great economic loss to our country . (3) also , as these resources are not inexhaustible , wasting them will be a treat to human's future surival . (4) besides , our waste will undoubtedly burden our family and society heavily . (5) therefore , it is high time for us to take effective measures to climinate waste on campus .
2.说明好处段
说明好处段一般出现在说明现象类作文的第二段或是利弊分析类作文的第一段或第二段（具体根据提纲），其基本的句型构造是：先用一句话总述问题的好处或优点，这是段落的主题句（如果出现在第一段，该句前面一般还有一句对主题的总述）；然后用3-4句话列举2-3点具体的好处或优点，以支持主题句。
例
directions : for this part , you are allowed 30 minutes to write a composition on the topic the pros and cons of college students' taking part-time jobs . you should write at least 120 words following the outline given below in chinese :
1.分析大学生做兼职的利与弊。
2.谈谈你的看法。
第一段：说明好处段
(1) nowadays many college students take part-time jobs in their spare time . (2) it is no doubt that taking a part-time job brings about several advantages to college students . (3) above all , part-time jobs provide students with a valuable chance to put their knowledge into practice . (4) moreover , the experience they get from part-time jobs will preapare them for the future job market . (5) besides , students can get some money to help unload the burden on their families .
除了说明危害段和说明好处段以外，四级作文中还经常涉及说明影响段（大多出现在说明现象类作文中），其结构与说明危害段和说明好处段基本相同，影响的内容可能有好有坏，也可能属于中性（无所谓好坏），考生在具体写作时要注意灵活处理。