Perhaps we were talking about a bear. "What is the bear's breakfast?" "The apple is its breakfast."
Well that won't work, the bear is common gender "Bjørnen" and we would have to use "dens". The animal would work "et dyr" and then it could be the bear after all!
The apple is the animal's breakfast. (Little do they know which animal it is!)
(Æblet er et dyr morgenmad.) Wait, I don't know how to do possessive with nouns yet.
Æblet er dets morgenmad.
Ok, just to make the things clear to me. Sorry for my possibly bad Engelsk.
Hans/henes - if we talk about something that is possessed by a person only (a man/woman) Dens/dets - they must not be as widespread as "hans/henes" because they are used when we talk about something possessed by a thing/creature/animal etc. (not human). Because you rarely talk about things that are not held by a human (f.e bear's beer, duck's newspaper)
Am I right?
In the case of sentances they have showed us so far, it doesn't matter if you use dets or dens. They don't mark either one wrong. It probably isn't so important unless you are writing more than one sentance. For example, if I am just talking about "it" and it doesn't matter what "it" is, I can use either one, but if I were to say "Min kat drikker. Den drikker mælk." (My cat drinks. It drinks milk.) it would be important to use "den" refering to "kattEN" (the cat) This is why it is important to know both.