"You are eating that fish."
Translation:Jecie tamtą rybę.
Basically, the determiner has to match the grammatical gender, number and case of the noun it describes. Take a look at what alukasiak wrote here: https://www.clozemaster.com/blog/polish-demonstrative-pronouns/
Yes, it's still "mysz". Hmm... it's already an exception (looks masculine, is feminine), so I guess its Accusative is another exception here.
"myszę" would be the Accusative if the basic form was "mysza" (which some people use, but it is colloquial at best, if not just wrong). Actually, other cases (apart from Vocative) do look as if the Nominative was "mysza". Strange word.
I remind myself that it doesn't change by associating it in my mind as being like "deer" and "fish" in English in that the singular spelling and plural spelling are the same. I know we aren't talking abt plural vs singular in the Polish in this case, but it still helps me to remember that "mysz" is "mysz" is "mysz" :)
Both are feminine singular forms, "tamta" is Nominative and "tamtą" is Accusative.
No. "you" has the gender of the person you are referring to. Which doesn't matter here (Present Tense verbs), but it may matter in some other cases. For example with adjectives. Imagine saying "You are tall".
If you're talking to Adam, you will say "Jesteś wysoki".
If you're talking to Anna, you will say "Jesteś wysoka".