"My neighbors are not poor."
Translation:Moi sąsiedzi nie są biedni.
From the rules for adjective declination: masculine personal plural a) soften the stem consonant b) add "y" or "i"
So biedny - (softening) - biedn' or biedni - (add y or i) - biednii or biedniy
I guess those last two look so unnatural that they just shorten either of them to biedni
Exactly as immery said. The only time you could see „ć”, „ń” or „ś” before a vowel is when it's used in a Sanskrit loanword (and then not by all writers). Otherwise those sounds are written as „ci”, „ni” and „si”.