"My mother is a cook."
Translation:Moren min er kokk.
In Norwegian, there's two different ways to write "my mother". Option A is "Min Mor", and option B is "Moren min". Both are equally valid, but option B is more common.
As for why this is.... Unfortunately, language does not always conform to our expectations, and sometimes they have unexpected or unusual rules. You'll unfortunately just have to get used to using the definite form when using option B.