"This policewoman is new."
Translation:Ta policjantka jest nowa.
Być requires the instrumental only when you have two nouns (Policjantka jest kobietą.), but here you have a noun and an adjective.
I have a question to native speakers, though: What happens if we use adjectives as nouns? What I have in mind:
They are poor. = Oni są biedni.
They are the poor. = ???
"The poor" literally translates to "biedota", so you can say:
- "Oni są biedotą" or "Oni stanowią biedotę" ( = "They are the poor"),
but the word "biedacy" (singular: "biedak") sounds even better to me:
- "Oni są biedakami" ( = "They are the poor").
What about the adjective "nowy"? Honestly, I don't think it's possible to apply the same transformation in this case, we do, however, have a very, very informal word "świeżak". The noun originally denoted a person who had just went to prison, and now it means a person who has just started a new job, a student who has just been transferred to another school, etc. You can say:
- "Ta policjantka jest świeżakiem" ( = "This policewoman is new (replace the word new with an appropriate slang word)")
Keep in mind that this noun is informal, and some people might find it offensive.
I've been thinking about it and unfortunatly could not find any rules.
We have rule that if the noun is "like adjective" - służący, królowa, średnia it follows adjective declension, but ona jest królową, on jest służącym.
I think the poor/the adults would work with "tymi"
On jest tym nowym, oni są tymi dorosłymi. It works then.
"policewoman" is more of a newly invented idea. Words that end in -man are not gender specific, but feminists use it as an example of sexism. I don't know if I agree with that or not, but these are very rare terms that have only been coined recently. It is so rare, in fact, that Google tries to correct it if you search it.