To answer a question you just click the green box (but that's only if two people haven't already answered it)
To comment on someone's answer you just click the small gray under the persons answer. If the question that was asked is old, then the comment box won't be there.
And to ask for more infomation, but without having to answer the question, you just go below the question (Where it says 'Ask for deatils, Follow, report') and just click Ask for details.
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."
