Well, you don't exactly comment per se. You click "answer" or go to someone else's answer and click reply (not sure the exact labelling but it's something along those lines).
Edit: It says "add your comment to this answer here", not "reply."
The correct sentence is: Two of the best singers were them. This makes sense because when you rephrase the sentence, "them" becomes "they" which appropriately describes the nouns of the sentence. The modified sentence then becomes: They were the best singers.
I would love to help but I am not good with analysis but you should read the poem if you haven't already because it is good :-)
