Note that this is not a normal thing to say in Swedish. The normal expression would be Jag äter fisk.
The sentence here makes me visualize a person with a smug face standing next to an open aquarium.
I think I'm going through these lessons too quickly, so words are starting to blend together. Let's get this straight:
Fisk is singular.
Fiskar is plural?
Fisken would be "the (single) fish?" Fiskarna would be "the (plural) fish?"
Yeah, most plurals end in "-ar, -er, -or". Most definite forms end in "-en, -et". Most plural definite forms end in "-arna, -orna". But that is simplifying things.
I found the system confusing at first but after a week or so it began to make sense and I actually prefer it to how things are in English now.
I'm not sure if you're on mobile or not but if so then you should take a look at the lesson tips and notes on your laptop/desktop (they don't show on mobile) as they go into more detail and explain some of the rules and such.