命题作文要求就所给的作文题目进行写作。与其他作文相比，命题作文具有较大的自主性。因为同样的一个题目可以从不同的角度、用不同的文体来进行写作。例如“My Teacher”这个题目可以写成一篇记叙文，也可以写成一篇描写文。不管用什么文体，也不管从哪个方面来写，只要写出的文章切题、流畅、条理清楚、语言正确就是一篇好文章。然而，这种较大的自主性也会使考生感到困难。因为在有限的30分钟之内，要形成提纲，并写成短文并不是件易事。这就要求考生在写命题作文时首先一定要审好题，要仔细分析题意，搞清楚题目所涉及的范围，确定文章的中心思想，切忌跑题。再者，要选择正确的文体。最后，要围绕文章的中心思想，将写作素材进行分析、归类并写好写作提纲，包括引言句、主题句和结尾句。
[例]
Fake Commodities
Markets all over the country are swamped with fake and poor commodities. This has harmed greatly the states interests and peoples health. For example, false poison made in a factory not only cant kill mice, but also gives them immunity and makes them healthier.
The cause of this serious situation is that some enterprises only want to get benefit. What’s more, some local governments, for the purpose of local protection, don’t investigate such activities, on the contrary, they even support them. The judicial organs don’t punish the offenders strictly.
We hold that the whole society should pay greater attention to these illegal activities, and these illegal activities should be punished according to the law. The leaders in charge should take effective measures to stop these harmful activities. Consumers should learn to protect their own interests, and not to buy fake commodities. In this way, we can at last wipe out fake commodities and leave no room for them.
作者把这个题目写成了一篇说明文。一开始先指出了假冒伪劣产品的危害。接下来分析了这一情况产生的原因，最后呼吁全社会都来关注这个问题。
摘自：《大学英语四级考试写作专项突破》
