They're not. "Słońce" dates back to pre-Indoeuropean "saul", and Latin 'sol'.
"Słoń" is an effect of many misunderstandings: Latin 'elephantus' turned into Gothic 'ulbandus', and that got to Slavic countries to become Polish "wielbłąd" - camel. Deprived of its name, 'słoń' stole the Turkish name for a tiger: "e[r]slan".
Do sentences like "To jest słoniem w pokoju." exist in Polish (with the English meaning of "that is a seriously important subject which we prefer to ignore and will not talk about right now."?
Personally I have never encountered "słoń w pokoju" in Polish and if I heard it, I would just consider it a calque of the English idiom. It does return 4000 results in Google, though. Polish Wiktionary has an entry for an idiom "słoń w salonie" (elephant in the living room). Frankly, I haven't heard this one either, but it has 13 700 results. So I guess the chance that you'd be understood is pretty big, whether your interlocutor knows it as a Polish idiom or just understands the English one. I wouldn't recommend it, but it's just my personal opinion.
As grammar is concerned, you generally shouldn't use Instrumental in "This is Y" sentences, unless this "to" is something... something that you absolutely don't know how to call. To me it sounds as if you saw an alien life form and had no clue what word to use to refer to it. In that way you kinda use "to" as some weird noun. So basically... let's just say that you shouldn't do it, period ;)
P.S. Even the first page of results for "słoń w pokoju" mentions the English idiom, and the first page of results for "słoń w salonie" gives links to photos of an actual elephant in an actual living room. OK, an artificial elephant, but still.
Thanks. I oscillated for some while between "słoń" and "słoniem" and finally settled on the wrong one :-(
Is There a difference between "Drinks" and "is drinkin" in polish? For both options seem to Count
99% of Polish verbs work perfectly fine for both Present Simple and Present Continuous.
Those exceptions are Verbs of Motion and a few verbs that also have habitual forms. There is a habitual form of "pić", it is "pijać". So theoretically "The elephant drinks milk" could also be "Słoń pija mleko". But that's not in the scope of this course. The normal verb works very well here.