This is the 6563rd most frequent Chinese word.
日文
Japanese language.
In this sentence, '日文' is used to refer to the Japanese language as a subject or field of study.
我在大学里学过日文。
I studied Japanese in college.
Here, '日文' modifies '书籍,' indicating that the books are written in the Japanese language.
日文书籍对我来说比较难懂。
Japanese books are relatively difficult for me to understand.
In this instance, '日文' is used to describe the proficiency or level of skill in the Japanese language.