Correct me if I am wrong but is the accusative form being used with the word koszulę becuase wearing the shirt is an action
Isn't "you are carrying a shirt" technically right also (for instance, by hand)?
in the "carry" meaning nosisz/niesiesz is like iść/chodzić - different verb for right now (iść, nieść) and for usually (chodzić, nosić).
"You are carrying a shirt" is "niesiesz koszulę"
"You carry a shirt" is " nosisz koszulę"
I don't know if it's accepted, because without more context this sentence is much more likely to mean you wear a shirt. (less likely "are wearing")