This is the 5167th most frequent Korean word.
놓았다는
"놓았다는" means "that (someone) placed/put."
The word "놓았다는" here is used to indicate a reported statement about the past action of placing (놓다) the book.
그는 책을 테이블 위에 놓았다는 말을 했어요.
He said that he placed the book on the table.
The word "놓았다는" is utilized to describe the action of having placed (놓다) the keys inside, which is the subject of realization.
주차된 차의 열쇠를 안에 놓았다는 것을 알게 되었어요.
I realized that I left the car keys inside the parked car.
In this sentence, "놓았다는" refers to the past action (of placing) as reported or described, which is complimented.