This is the 10151st most frequent Turkish word.
başladıklarını
"Their starting" or "that they started."
In this sentence, 'başladıklarını' is used to express the action (starting) of the students (subject of the subordinate clause) being recounted in an indirect quotation, with 'duydum' indicating I as the speaker heard this information.
Öğrencilerin yeni projeye başladıklarını duydum.
I heard that the students started the new project.
Here, 'başladıklarını' is employed in the reported statement to indicate what 'they' were claiming about their actions regarding planning a vacation.
Onlar tatil planları yapmaya çoktan başladıklarını söylediler.
They said that they had already started making vacation plans.
In this example, 'başladıklarını' forms part of the subordinate clause, conveying the information the team members shared about their actions.