One of the correct translations is "Isso é uma caneta" Is that really correct? I thought isso could only be used when the subject is not mentioned or said specifically.
Also, why can I not use "Essa é uma caneta?"
Isso é uma caneta is correct as well. You have been looking at a strange thing and ask yourself: "what is that?" You go there, pick that thing and say:"isso é uma caneta" :)
Why not "aquila"? Shouldn't the genders match?
"Aquila" does not exist. The correct word is "Aquela".
Oops, that's what I meant! So, are you correcting just me or Duo's answer here and me?
hahahaha..."Aquela é uma caneta" is also right, but I think I'd use "Aquilo..." anyway. It's used more often to explain what an object is.
So is "aquilo" used for objects regardless of their genders?
Yes, "isto", "isso", and "aquilo" are neutral.