"That fish is eating a sandwich."
Translation:Tamta ryba je kanapkę.
"Ryba" is a feminine noun so the proper determiner is "tamta". You would use "tamto" for neuter nouns.
It's not an accent mark. It's a totally different letter, a nasal "o" in fact.
Words like "Tamta", "ryba", "kanapka" are Nominative. The basic, dictionary forms.
"tamtą" can actually be either Accusative or Instrumental (usually feminine word in Accusative has -ę ending, like 'kanapkę' here, but "tamtę" doesn't exist), but mostly, a word ending with -ą is a feminine word in Instrumental.
Why don't you have "lessons" (like the ones for spanihs) where you expain different grammar and lexical basic ideas?
I think you mean Tips&Notes. Polish has them written until the "Prep." skill, but I don't know if they are currently accessible via app. They are surely accessible via the browser version.
I think Spanish may be different because the Tips&Notes are made by Duolingo staff and they are constructed quite differently. I'm not sure though.