This is the 545th most frequent Korean word.
시작한
"Started" or "that started."
Here, '시작한' is a participial form describing 'the state of having started'.
그는 새로운 프로젝트를 시작한 상태이다.
He is in the state of having started a new project.
In this sentence, '시작한' is used as a modifier for 'event', indicating it was started by 'they'.
그들이 시작한 행사가 큰 성공을 거두었다.
The event they started became a great success.
Here, '시작한' modifies 'book', specifying that it was the book the speaker began this morning.