This is the 13407th most frequent Korean word.
떠난다고
"떠난다고" means "saying that (someone) is leaving" or "will leave."
In this sentence, '떠난다고' is used to express a reported speech of future departure.
그는 내일 떠난다고 말했어요.
He said that he will leave tomorrow.
Here, '떠난다고' serves to introduce the reason for surprise due to the announcement of leaving.
나는 그녀가 떠난다고 해서 놀랐어요.
I was surprised when she said she was leaving.
In this instance, '떠난다고' is used to convey the information received about someone's departure.