"This is our common room."

Translation: To jest nasz wspólny pokój.

[va-diim]

I understand that the nominative case is used with

To nasz wspólny pokój.

But, is the instrumental case incorrect with

To jest naszem wspólnym pokójem?

[Jellei]

Yes, it's incorrect. The sentence has the template "This is Y", and such sentences just start with "to", which is then the subject.

[va-diim]

I don't understand the whole "X is Y" thing. If "this is Y," is in the nominative case, then how does "Y" depict nominative case, as opposed to "X is Y"? X is Yiem?

I understand when to is used without jest, są, etc., then the noun that follows stays in the nominative case. So, the verb jest always precedes nouns in instrumental case except when to precedes jest? Then, the noun that follows also stays in nominative? Or is instrumental sometimes correct after to jest?

[Jellei]

Well, I guess it's not a surprise that I will just link to my topic here, it's hard for me to say anything more about this matter which I haven't said yet ;)

"jest" is followed by Instrumental in most cases, but not all of them.

Please take a look at parts one (the general one) and four (This is Y).

[va-diim]

OK, thank you. So I understood that the jest is optional in "to jest," so the Y stays nominative.

[anya_mz]

"To jest nasz pokój wspólny" should be accepted.

[Jellei]

OK, added.