"Naszą planetą jest Ziemia."

Translation: Our planet is Earth.

[Kulink]

So in this sentence 'our planet' is in instrumental case and 'earth' in nominative case.

Can it be stated reversing the cases?
Like 'nasza planeta jest Ziemią? Or is that not allowed with certain types of nouns?

[Jellei]

I made an attempt to write something about the problem you mentioned here, part 1,5.

Even if that could work for "an Earth" as Emwue said, because it can be argued that's some type of a planet, that wouldn't work for "Mars" for example. Mars is a planet, a planet is not "Mars".

[Jean-Lucfranois]

The link "here" doesn't work

[JerryMcCarthy99]

Try this, which will take you to the same place: https://forum.duolingo.com/comment/16373167

[Daniel736034]

So I completely understand that.

You cannot say "a planet is Earth." And it appears that is the reason you could never have the cases swapped?

Similarly you can say "Lew jest kotem." But are you saying the phrase "Our cat is a lion" can only ever be constructed as "Naszym kotem jest lew" and never "Nasz kot jest lwem"?

Or is there some difference between the two examples?

[Rysiek141]

If it turns out that the aliens speak Polish, this sentence will come in very handy