I think there is some of weakpoint in Dulingo which is : you can't get the full Rule for everything .. you just hear , Read , and try to extract the Rule by your own understanding and maybe you go wrong with that ..for example: 70 % of Turkish learners who are in advanced stages here have no Idea about : the ketchup Rule or festuk şehap Rule or the phonetic Rule for o ö u ü a ä at the end of each word etc .. please keep it in consideration
onu is the accusative case of o.
You also can build the accusative case of ben and sen:
ben -> beni
sen -> seni
beni / seni / onu are only different concerning the endings due to vowel harmony.
Generally the endings (suffixes) of the 4-way-vowel harmony can be u / ü / i / ı depending on the vowel before.
So it is:
oh my god this is like the 58 thousandth time that im attempting to test from 4 to 5, but i keep losing cuz i make stupid mistakes xD
heres one: i forgot peynir was not peyniri so i decided to apply the accusitive form to the existing accusative form and ended up with peyniriyi. Duolingo was not impressed