"A lot of us are old."
Translation:Wielu z nas jest starych.
i am confused with singular and plural here. "wielu z nas" seems plural and starych is confirming this. But why then are we using "jest" and not "są" here?
In terms of the 'być' verb, such words as 'mało', 'dużo', 'wielu', 'wiele' and so on take the singular form, even though they denote plural. I understand why it may be confusing, but for a native it's hard to even imagine "Wielu z nas są starych" or "Wielu z nas jest stary", that's just wrong on so many levels.
gramatical gender. and cases.
Wiele = feminine nouns, neuter nouns, masculine nouns that do not describe a person (nominative, accusative , wielu- genitive,dative, locative, wieloma- instrumental)
Wielu= male persons. (all cases)
I have no idea about mixed gender persons group, as my gut disagrees with my brain. (according to sjp.pwn wiele ludzi - many people is old-fashioned, and wielu ludzi is more proper)
What case is "starych" here? Not nominative. Is it genitive to agree with "nas"?
Yes, it's Genitive, "wielu" and most (if not all) quantifiers take Genitive.
Since nas is already genitive, what is the purpose of adding the "z"? Would z be used in all instances where wielu is used with a personal pronoun?
Well, you have "a lot of people" = "many people" = "wielu ludzi", but it's something a bit different here. You can't substitute "a lot of us" with "many us", right? It's indeed "many of us". "[a lot of/many] out of us". That's also why you need the "z" in Polish, because it is in fact something a bit different grammatically.