"This gentleman is tall."
Translation:Ten pan jest wysoki.
The masculine singular ending is -y, as in dobry good, ładny, pretty, miły nice, kind. This ending is spelled -i after k and g: wielki great, drogi dear, expensive. The feminine singular ending is -a, as in dobra, ładna, miła. The neuter singular ending is -e, as in dobre, ładne, miłe, spelled -ie after k and g: wielkie, drogie.
You have the point. I've never thought about it. It looks like velar stops (k,g) always go with -i instead of -y (e.g. szybki, długi).
I have no idea why it is like that and I can't see any pattern. Every other consonant seems to go with -y but out of the blue you have -pi instead of -py (e.g. głupi; but gruby). Maybe there's some reasonable explanation but for me it appears to be completely random.