说明文是以说明为主要表达方式来解说事物、阐明事理而给人以知识的文章。它通过对实体事物的解说，或对抽象事理的阐释，使人们对事物的形态、构造、性质、种类、成因、功能、关系或对事理的概念、特点、来源、演变、异同等有所认识，从而获得有关知识。如我们学过的Christmas Day，The GREat Green Wall，The University and Man-made satellites，Standing Room Only等都是很好的说明文。
有人说，说明文的意思就是解释，解释就得把话说清楚明白，做到“说明说明，一说就明”。因此，在写作上有个重要的要求，就是语言准确简明、通俗浅显、条理清晰而有逻辑性。此外，还要抓住事物的特征，把握说明中心，选择合适的角度。另外，为了做到表述清楚，说明文的层次必须分明，为此，一般各段都有一个主题句，并且往往放在段首，以引起读者的注意。为达到以上这些目的，需要做到下列几点：
1．限制说明的事物和范围，因为不可能在一篇短文中解释太多的事物。
2．准备足够多的、确切的例子，进行阐述、例证。
3．用正确、恰当的顺序摆出事实、观点。
4．注意词和句子的准确性。
5．尽可能使说明的内容有趣、生动。
参考范文：
Why I Am Learning English
English is one of my subjects at school．I like it very much．I have learned it for nearly three years．It is not easy to learn，but it is very important for us．
English is the most widely spoken language in the world．People in different countries often use English to talk with each other．Our country has opened its doors to the world．More and more people come here as travellers or on business．So English is getting common in our country．Of course，I must learn it well．
English will be useful to me after I leave school．If we want to do business with foreigners，we must know English，because most of them speak and write English．In the world，most science books are written in English．I am learning English so that I can learn much knowledge．After I have learned English，I can read short stories in English．I can also sing many English songs．English helps me enjoy myself after class．
分析：本篇文章简明扼要地从四个方面介绍“我”为什么学习英语，即English is one of my subjects at school．English is the most widely spoken in the world．English will be useful to me after I leave school和 English helps me to enjoy myself after class。从而说明了英语的重要性。
全篇四个自然段，在前三段中，每段的第一句话为该段的主题句，其后均围绕着这一主题句展开。最后一段的主题句在段末。整篇文章条理清楚而富有逻辑性。
