So confused by this - why not Nejsi stari
The above (nejsi stará) is feminine. Masculine is nejsi stará, plural (masc. animate) nejste staří. See the tips and notes about the hard adjectival declination.
I’m pretty sure that in Moravia people say nejseš not nejsi
Actually, nejseš comes from Central Bohemian dialects (now Common Czech) and only later came to Moravia.
We teach standard Czech here, not any dialects. In the very last skill of the tree we show some Common Czech.
Thanks for the supplementary info, though. :)
why not Nejsi stary?
Nejsi starý! is the masculine version and is also accepted. Nejsi stará? is feminine.
Why isn't it ty není staré?
You mean for neuter? Like Ty nejsi staré (dítě)? It is super weird to address anyone in the neuter gender. But even that would require NEJSI or NEJSTE for the second person.
Otherwise I can't see any other reason you might have in mind.