why can't i say "o quarto" = the room, also?
That should also be accepted!
Would it still be correct to say ¨uma nova cortina¨ and have this particular adjective in front of the noun?
Yes, the same sense!
I wrote "precisa uma" forgetting the "de", is that really correct or is a gift from duo seeing I had no heart left ?
It seems you got a gift then =)
I also wrote 'precisa uma' by accident, but it's not correct then?
Yes, that's wrong...