공부하는 is the act of studying so "studies" In this sense 공부한 is not shortening or modifying that... It is actually making it past tense.
I think more reflexive pronouns should be acceptable translations in this situation. The student that studied. The student who studied. Etc
Then just report it by flag.
"The student that studied is very smart." is now accepted.
Should it be studies, not studied?
공부하는 학생 would be "the student who studies"
Adding -은 makes it past, 는 is present, 을 is future.
공부한 학생은 아주 똑똑해요 as it is studied, why not 공부했던 학생은 아주 똑똑해요?
The translation would be valid, but there is difference in nuance.
"공부했던" implies that the student studied in the past but is done with studying. "공부한" is a simple statement that the student studied in the past.
I translated it as "The studied student is very smart." and it was counted incorrect.
"The studied student" isn't correct English. It should be "the student who studied"
Same, and Google Translate gives the same Korean for both. shrug