I asked a Czech friend, who didn't really know, but we surmise that the 'to' is needed because without it when using the third person it isn't clear who we are speaking about.
First person, it's clear:
Jsem stará žena - myself (there's only one me)
Jsme staré ženy - us (there's only one us)
Second person, it's clear:
Jsi stará žena - you (there's only one you)
Jste staré ženy - you guys (there's only one group of you)
Third person, it's not clear which woman/women we are talking about:
Je stará žena - a specific woman or another unspecified woman?
Jsou staré ženy - some specific women or other unspecified women?
We need to add 'that' (to) or 'they' (ony - at the beginning) in order to clarify who
In giving the English translation as an answer it's not needed to add anything, such as 'They are those old women' because in English we don't omit the personal pronoun (they) as is done in Czech.
Thank you Mattvick and others, but to me, whose native language is English, your explanation doesn't entirely make sense, to my native English speaking mind, anyway : ) Here's why: I fail to perceive how the "to" adds clarity, as you claim. That word ("to") is also vague, it's just a demonstrative pronoun like "those" in English. In this sentence, clearly we're referring to women, so we know we mean people and not things, even without the "to."
To my native English language mind, "they are old women" would be "jsou stary zena" and "those are old women" might be "jsou to stary zena" or, as I (native English speaker) would naively expect, "to jsou stary zena."
What I infer from other comments here, is that "jsou stary zena" just sounds strange to native Czech speakers and it's simply not said that way. And I can understand if that's the case- every language has its idiosyncrasies.
By the way, can one say "To jsou stary zena" or would that be wrong for some reason, or just sound odd, or mean something else?
Hi, "staré ženy" is description. Even though you say "Jsou staré ženy" I'd immediately ask "Who or what are they?". So you need to clarify "To jsou staré ženy"or "Ony jsou staré ženy", "Babičky jsou staré ženy", etc. I think grammatically I'd say that you can skip subject but object needs to be clear, not sure though
What if you were to say "Ony jsou staré ženy" or "Jsou ty staré ženy"? Is "to" a demonstrative or personal pronoun in this context?
Also, the Wikipedia page on Czech Declension mentions that "to" could be used such that this means "These are old women". Would that be correct?