What's the difference between weź and bierz in the imperative? i.e. how come it's "weź coś" but "nie bierz tego"?
It's safe to say that negated imperatives are always imperfective.
Can someone explain the grammatical difference
"Czekaj tutaj, nic nie bierz" and "Czekaj tu, nie bierz niczego" which was also accepted
There is no grammatical difference. Nic/niczego are interchangeable genitive forms. I would say that niczego is a bit more emphasized, but the difference is very slight.
Czekajcie tutaj, nie bierz niczego is wrong because of the placing of "niczego"? or the form "czekzjcie"? Or?
The problem is that you are talking to more than one person (czekajcie) and then suddenly just to one (bierz).
Oh, yes, of course!! Thank you!