"Write an essay" 是一种常用的表达方式，用于指示某人在书写一篇文章。这个短语起源于英语中的 "essay" 这个词，它的意思是"短篇陈述性文章"。

例如:

"I have to write an essay about the causes of World War II for my history class."
(我必须写一篇关于二战原因的文章给我的历史课。)

"The assignment is to write an essay on the impact of technology on society."
(作业需要我们写一篇关于科技对社会影响的文章。)

另外, 也可以说 “compose an essay”, “write a composition” 来表示写作文。
例如:

"She was asked to compose an essay on the topic of environmental conservation."
(她被要求写一篇关于环境保护的文章。)

"He was assigned to write a composition about his summer vacation."
(他被要求写一篇关于暑假的作文。)