Why isn't it possible to say "ta ryba"?
Are there any clues to gender or is it just learn as you go?
At the end of this post there is: "How to guess a gender of the noun?"
is this example with Nominative case?
Difference between ryba jest duże/ duża both accepted, why?
No, 'duże' cannot be accepted here as it's grammatically wrong. "ryba" is feminine, so it's 'duża'.
Why "tamta ryba to duża" is bad?
You can't use the "to" construction if it's followed by a standalone adjective. It's kinda like saying "That fish = big". "That fish = a big animal" makes some sense, but not the first one.
See here, especially part 3: https://forum.duolingo.com/comment/16373167
i put TA ryba and it says green (ok)?
but on the bottum it tells met TAMTA which i should have used
i wonder why i get the green light for a wrong answer ;)
I thought jest took the instrumental case?
If it's followed by a noun phrase. E.g. "That fish is a big fish" would be "Tamta ryba jest dużą rybą".
But if it's followed by a standalone adjective, it takes Nominative: "Tamta ryba jest duża".
What is the difference between "duże" & "duża"?
There's always a bigger one.