I was marked wrong for writing "fish is on the table." Would this be a correct translation?
Yes, it is a correct translation. I think this choice will be added soon.
Using the singular verb "is" requires an article (a, the) before "fish" - "the fish is on the table" (which could mean one fish or a mass noun group of several fish.) If you don't use the article, then you'd have to say "Fish are on the table".
So is there a difference between one fish and multiple fish? "The fish are on the table" was marked wrong. So would "The fish are on the table" be "Рыби - на столе"?
Yes, there is a difference:
if you can count it - one fish - "рыба", from two to four - "рыбы", five and more - "рыб". For example -fifteen fish - "пятнадцать рыб", give me three fish - дай мне три рыбы.
If a lot of fish, you may say - много рыбы (a lot of fish). If you want to say "many fish" (many types of fish) - много рыб.
But if you can't count the fish (or you don't want to count) - correctly to use in the singular. For example: Рыбаки пошли ловить рыбу сетями - the fishermen went to net fish.
So "The fish are on the table" would be the same as one fish - "Рыба на столе". Next time press report -> my answer should be accepted.
Also, рыба/рыбы counter is relative to unity digit, so you will say двадцать две рыбы, тридцать пять рыб
Is this rule - about the quantity vs diversity valid for things other than this particular case? Like: много сыра - A lot of cheese; много сыров - A lot of cheeses?
"The fish are on the table" would not be the same as one fish. Are is a present plural tense. "Рыбы на столе."
Since there isn't a direct translation for "there is," would "There is fish on the table" be correct, or would that be "na stole ryba"?
You are right - "there is/are fish on the table" would be "на столе рыба", it also means "рыба на столе", because in Russian language you can change the words in some places without losing the sense of the sentence.
That's what I tried as well, but it wasn't accepted (15 Dec 2015). I reported it, so let's see.
These are two different sentences:
In the sentence from the task one tell - "where is/are the fish lying". In your choice you just say - here's a kitchen, there is a table on the kitchen, there is/are fish on the table, there is a window next to the table