Translation:The fish was slowly swimming in the river.
So I guess we can't put "The fish swam slowly in the river" because we need to put 'swimming' due to the ”でいました”？
is that how you can tell if it's plural or singular? i'm having a hard time figuring it out.
It's entirely up to the author of a sentence whether they want to give you that information. If they ommitt it it probably wasn't anything special or just not important, or you already know it from context.
Regardless of the word order in English, it still is said the same in Japanese.
I put: fish were slowly swimming in the river. I dont get how this was wrong
It's not. They just don't have enough acceptable English answers.
It shouldn't be wrong when ました means past tense structure. Was and were in this sentence should be accepted.
For the listening exercise, it feels odd that answers in pure kana gets rejected. You can't hear kanji. When some earlier listening exercises rejected answers with kanji, it's very frustrating. It's even more frustrating when it can't be accurately reported, since there is no "my answer should be accepted" option.
It could be "the fish", "a fish", "fishes", or whatever other English term you could come up with to refer to 1 to infinity of fish.
Fishes is not the plural for fish. It's simply just Fish for multiple fish
I used the word 'swimming', and slowly can be placed at the end of the sentence
There are many more ways to translate this sentence into English. Word order is extremely important; It's not just about picking the right words.
Plurality is irrelevant in Japanese
Polite Japanese is not very direct, nor is it specific. It's less confrontational to speak in such a way.