This is the 3478th most frequent Japanese word.
真面目
Serious; earnest; diligent.
In this sentence, '真面目' (まじめ) is used as an adjective to describe the quality of the student being focused and dedicated in their studies.
彼は真面目な学生です。
He is a serious student.
Here, '真面目に' (まじめに) operates adverbially to modify the verb '仕事をする' (to work), emphasizing the manner of performing the task.
真面目に仕事をしてください。
Please work diligently.
In this instance, '真面目な' describes '顔' (face) as intense or focused, often capturing a figurative aspect of being earnest or sincere.