"A bird is eating a mouse."
Translation:Ptak je mysz.
„Myszka” is a diminutive form of „mysz”. The standard form is „mysz”, which in accusative is also „mysz”.
If you wanted to note, that the mouse is smaller than most mice, or that it's a cartoon or a fairytale mouse – sure, you can.
Well, there is a word "jej" in Polish, and it's very common actually, but it has a completely different meaning. "Je" and "jem" are similar, but the former is for the first person, while the latter is for the third.
I thought the noun is supposed to change if it's in accusative unless it's an inanimate masculine noun?
Well, yes. So I guess this is an exception... it is an unusual noun after all, looking masculine but being feminine.
This is the reason why is not mysza? Because I was expecting it would end up with -a being animated and masculine.. So in reality it is feminine, did I get the concept?
"mysza" is an existing word, but it is considered colloquial, so we do not teach it nor accept it.
Yes, "mysz" is feminine even if it doesn't end with -a.