This is the 2970th most frequent Korean word.
추천한
"Recommended"
In this sentence, '추천한' modifies '것이었어요' to denote the object that was recommended by the speaker.
그 책은 내가 추천한 것이었어요.
That book was the one I recommended.
Here, '추천한' is used to describe '식당,' indicating the restaurant that was suggested by a friend.
친구가 추천한 식당에 가보았어요.
I visited the restaurant my friend recommended.
In this instance, '추천한' qualifies '논문,' signifying the paper that was endorsed by the professor.