This is the 16784th most frequent Turkish word.
zanneden
One who assumes or thinks.
In this sentence, 'zanneden' means 'considering' or 'assuming', and it modifies the noun 'genç', attributing the quality of believing or assuming something to the young man.
Kendini her şeyi bilen biri zanneden gençle tartışmaya başladık.
We started debating with the young man who considered himself as someone who knows everything.
Here, 'zanneden' is used to convey 'assuming', focused on the act of mistakenly considering something, applied to 'everyone'.
Onun yalan söylediğini zanneden herkes yanıldı.
Everyone who assumed that he was lying was mistaken.
In this example, 'zanneden' means 'assuming', clarifying that the neighbor thought someone was the thief.