"写日记"用英语可以说作 "keep a diary" 或 "write a diary".

其中 "keep a diary" 的来源是由 "keep" 这个动词和 "diary" 这个名词组成，"keep" 意为"保持"或"维持"，而 "diary" 意为"日记"，所以 "keep a diary" 的意思就是"保持日记"，表示写日记的意思。

例句：

I keep a diary every day.
(我每天都写日记。)
She has been keeping a diary for years.
(她已经写日记多年了。)

"write a diary" 的来源是由 "write" 这个动词和 "diary" 这个名词组成，"write" 意为"写"，而 "diary" 意为"日记"，所以 "write a diary" 的意思就是"写日记"。

例句：

I write a diary every day.
(我每天都写日记。)
She has been writing a diary for years.
(她已经写日记多年了。)