Die Lehrer freuen sich über den Erfolg ihrer Studenten.
What I don't understand about this sentence is, den Erfolg is accusative but then why Studenten takes ihrer, dative? I know that ihrer here is a pronoun but why is it not ihre Studenten, why not accusative? Is it about über, but if so how can I understand which object takes accusative and which one takes dative?
Thank you for your help.
I'm writing an answer even though the comments essentially already answered this. But it's still listed as unanswered.
Über takes the accusative: “Freuen sich über wen? – Den Erfolg.” (Happy about whom or what?)
But that Erfolg is further described using a genitive: “Wessen Erfolg? – Den ihrer Studenten.” (Whose success?)
You could conceivably say “Die Lehrer freuen sich über ihre Studenten und deren Erfolg.” This would be using accusative with both objects. But it has a slightly different meaning, as it implies the teachers being happy about their students by themselves, not only about their success.
