I know the usage of von, genitive case and possessive pronoun, but when and how should I use zu in the sense of possession?
Demonstration zum Weltfrauentag
Prüfung zum Steuerberater
Übung zu Wortarten
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None of your examples actually indicate a possession. The demostration is on the occasion of the International Women’s Day, the examination is to become a tax advisor and the exercise is concerning word types.
All three of these could also take von, but that would change the meaning in every case to something much closer to possession.
Demonstration zum Weltfrauentag Prüfung zum Steuerberater Übung zu Wortarten
As usual understanding what the actual word means in context helps a lot.
Übung zu Wortarten --> Exercise in regards TO word types (about)
Prüfung zum Steuerberater --> I don't know what that is supposed to mean.
Demonstration zum Weltfrauentag --> Demonstrate at/during the Weltfrauentag. (Time)