"Our neighbors rarely swim in the swimming pool."
Translation:Nasi sąsiedzi rzadko pływają w basenie.
Almost all Slavic languages borrowed it from French bassin. I couldn't check when this word first appeared in Polish, but the first mention in the Russian Corpus is from 1755, which is not surprising, since hundreds of French loanwords were known to enter the language in the 18th and 19th centuries.
"Our neighbours" are simply the subject of the sentence, so most usually they'll be put in Nominative (not that other options, like Dative, are impossible). Then you have a verb, and swimming pool is in Locative. I don't know what could be in Genitive here.
If you wanted to put them in Genitive, well, you'd need a verb that works well with Genitive, like many negations: "Naszych sąsiadów nie było dzisiaj w domu" = "Our neighbours were not at home today", or to translate the Polish version literally, "There were no our neighbours at home today".
Kogo/czego will rather be Whom/What.
I wonder why in Poland foriegners are considered a bit 'slow'. I have read many comments here by the creators of the course (a huge and difficult task) indicating that the students could be confused unless things are described fully. So we keep getting 'swimming pool' as the preferred choice, for example, instead of 'pool' even though it is obvious only swimmable pools are suitable for swimming. I mention this because 'basen' is also a bedpan, but no one suggests that Poles would be confused. So please, when making decisions, respect us more.