I may have seen something that would have answered this and forgotten already, but for feminine nouns with the s posessive, does it make a difference whether you use the -en definite or the -a definite? That is, could you say 'kattas mat' instead of 'kattens mat'? If you could, are there any particular differences between the two? (Edit: just came across “De leser boken for jentas skyld” in another lesson, which helps answer this, but will leave the comment up anyway.)
Different question: sort of wondering about the 'katta/katten sin mat' type of posessive as an alternative for the 'kattens mat' type... ok at all here, or best avoided?
'katten' and 'katta' are actually the definite forms of two different words (though most Norwegians don't treat them as such), rather than the masculine and feminine version of the same word:
katte (f) = female cat
katt (m) = cat (of any sex)
We're allowing both, as the context doesn't say anything about the cat being male here, but technically there is a distinction.
For words which can be both masculine and feminine, like 'jente' (m/f), there is no difference in meaning between saying 'jentas' (f) and 'jentens' (m), just try to be consistent about which one you use.
"katten sin mat" is a construction called garpegenitiv, which, while not being strictly incorrect, is often considered to be less proper than "kattens mat" or "maten til katten", and even frowned upon by some.
Generally I'd say that it sounds fine in short sentences regarding personal ownership, especially when spoken, but I would definitely avoid it in more complex, abstract or technical sentences - and if I were taking a Norwegian exam.
"Dette er Kari sitt hus." - perfectly fine
"Dette er arbeidskommisjonen sine hovedoppgaver." - not good