It is said "They catch fish" which is singular yet the answer has a -j which denotes plurality of the word. Thanks.
In English the plural of fish is also fish.
They catch a fish (singular) = ili kaptas fiŝon.
They catch fish (plural) = ili kaptas fiŝojn.
I did not know that, thank you
The word fish can be used very broadly.
You can also use it to describe multiple people fishing (e.g., they fish tuna). The word fishes can also be use as a plural for fish, but this normally indicates multiple species of fish.
I am sorry, the plural of fish is fish in English. If it was only one fish the English sentence would be: They catch a fish.
why the accusative n on fish?
Because there is an action directed to fish (object)