This is the 11164th most frequent Turkish word.
döndüklerini
"Their returning" or "that they returned."
In this sentence, 'döndüklerini' translates to 'that they had returned' and serves as the object of the verb 'fark ettim' (I noticed), indicating the subject's realization of their return.
Döndüklerini fark ettiğimde çok mutlu oldum.
I was very happy when I noticed that they had returned.
The word 'döndüklerini' means 'that they returned' and is used here as the object of the verb 'öğrendim' (I learned), introducing a subordinate clause indicating what was learned.
Onların İstanbul’a döndüklerini bugün öğrendim.
I learned today that they returned to Istanbul.
Here, 'döndüklerini' translates to 'that they had returned' and functions as the object of the verb 'bilmiyordu' (did not know), specifying the information that was unknown.