"You are drinking my cat's milk."
Translation:Du dricker min katts mjölk.
It is quite simple really. You can say either "du dricker min katts mjölk" or "du dricker kattens mjölk" but you can't say "du dricker min kattens mjölk" just as in English you can't say "you are drinking my the cat's milk" - the rule works the same in both languages in this instance.
Exactly... I chose "kattens" as well because I thought that the speaker means a particular cat. His/her cat, so it's by all means THE cat. :( I can totally get why we ommit "the" in English. It's a separate word. Ending -en is just a suffix that slightly changes the meaning of the word so I assumed it should stay in place whenever this altered meaning is meant. Swedes should sit and think about it... :(