For być and its conjugates to have instrumental, there must be two nouns (substantives) in the sentence, one the subject and one the objet. for example, "to jest rower" (It is a bicycle - only one noun, the object),compare with Joanna jest polką (Joanna is a polish woman, two nouns, subject - Joanna, object - Polka). (Many thanks to Joanna, my Polish teacher in Kraków who explained this to me.
Joanna is right, but there's one detail missing: subject pronouns are also valid here. "Ona jest Polką" = "She is a Polish woman" :)
Oh, and we do capitalize the demonyms for countries and continents, so that's why it's "Polka". Although we don't do that with demonyms for cities, so "Joanna jest krakowianką" (if she really is from Kraków, of course).
It is probably because butelka is not an object of the verb here, but subject.
Butelka is the object here, but the verb być is special, but does not require the accusative but the nominative.
Firstly, the object of być requires instrumental, and secondly, this sentence does not have an object.
I was confused here too. My guess is that it is because 'Gdzie jest' uses a different case than 'Ta jest', but I would appriciate clarification by someone here.
In "Ta jest kobietą", Ta is the subject and kobietą is the object. In "Gdzie jest kobieta", kobieta is the subject and there is no object. Gdzie isn't a noun, so it can't be a subject or object of a verb; Ta is a pronoun, so it can.
The subject of "jest" should be in the nominative case (kobieta) and the object should be in the instrumental case (kobietą).