This one I don't get. My grammar text says....
"The masculine personal plural adjective ending is -y/-i and the preceding consonant is softened: dobry → dobrzy, ładny → ładni, miły → mili, wielki → wielcy, drogi → drodzy; for more examples, see below."
Swan, Oscar (2008-10-12). Polish Verbs & Essentials of Grammar, Second Edition (Verbs and Essentials of Grammar Series) (Kindle Locations 780-782). McGraw-Hill Education. Kindle Edition.
Ok, but ż (or rz by its other form) is already soft, is it not? And z is actually considered a hard consonant, so here we're actually going in the opposite direction, from soft to hard!!!
well "ź" is as soft as our consonants get, but I tried finding few -ży ending adjectives and how they change, and boży, chyży, hoży, ryży, wraży all have masculine singular= masculine personal plural.
So all you have to do is learn this exception. (also please do not learn all those other words, I do not know what exactly they mean , other than Boży, which is God's )
The problem is, that it's not that simple. Or maybe it actually is?
"Duzi" is masculine personal plural. This plural is used almost exclusively for 'groups of people including at least one man'. As "people" clearly includes at least one man (otherwise it would be "women"), it uses "duzi".
"Duże" is not masculine-personal plural. Which is used for everything else. For apples, for spiders, for boxes, for women... everything that is not 'a group of people including at least one man'.
The ź sound (spelled just "z" before an "i") is [ʑ] while the ż sound (also spelled "rz") is [ʐ].
Neither of those sounds exists in English, so the two might both sound like [ʒ] to you (the "zh" sound as in "vision, measure").