"They are old."
Why then in a previous task there was "TO jsou stara zvirata"? Is there any difference?
yes, the difference is the inclusion of the noun "zvířata".
- To jsou stará zvířata.
- To jsou zvířata.
- To nejsou zvířata.
- Jsou to zvířata.
- Nejsou to zvířata.
are all ok, but
- To jsou stará.
is not. the only related sentence with a nounless adjective used predicatively that works would be
- To je staré.
i note that in that case the natural gender of "to" (singular neuter) matches that of the adjective, so in that last sentence we are really dealing with a slightly different "to".
an exceptional, very informal/expressive sentence that also works is
- To je stará.
where "stará" is not an adjective despite the appearances, but instead functions as a noun meaning the speaker's wife or girlfriend.
Why are here "Jsou stara" and "Oni jsou stare" possible? Why does stara has two different endings here? Shouldnt it be the same gender?
Masculine, mixed or unspecified group: Oni jsou staří
Feminine: Ony jsou staré
Neuter: Ona jsou stará
Thanks for the examples; they are very clear and helpful. A question: why does the "r" change in stari (sorry, my keyboard won't let me make the change) from "stare' " and "stara' " ?
it must change because R is a hard consonant unable to be followed by the required soft ending Í. R becomes Ř in such cases. three other such shifts: H into Z, CH into Š, K into C.
And we know that we need to use "oni" instead of "ony" because "staří" is masculine, correct?
It depends on the gender. I suggest you to read the Tips and notes or any other resource about the declination of Czech adjectives.
Read also the previous discussion here.
Why isn't "Jsou oni staří" accepted here, when "Jsou ony staré?" was accepted for "Are they old?"? Does the question form allow for a more flexible grammatical placement?
Czech is an SVO language, use a different order only when you have a specific reason. A question often uses the VSO order, unlike a normal declarative sentence. It is not natural for a declarative sentence.
The SVO order is: Oni jsou staří.
I'm having difficulty knowing when to use the pronoun (oni in this case) and when not. In some cases it doesn't seem to matter, using or not using appears interchangeable... But in other cases without pronoun it is talking generally about all the old people in the world, vs. These specific people being old.
Sorry for my confusion, thanks if you can give me some insight. So far I've learned a lot by asking questions!
That's really too broad. Ask about some particular sentence you have a problem with. This one can is easy and both are possible.