Can you only add comments to questions you have answered?

Look UNDER any resolved question. There should be a link in the toolbar under it to post a comment. The problem is, nobody receives a notice of this posting -- and almost nobody goes back to their old questions and looks for these comments.

[EDIT: the comment link is the 3rd one, after STARS & EMAIL]

When you are looking at the resolved question, down below the Best Answer box you will see these four tabs...

Interesting Email Comment (0) Save

Just click on [comment] and leave whatever information you would like to share

Everybody was given a chance to give comment on resolved questions. But of course, they can also report rude comments.

Your comment also serves as answer and suggestion to a certain question.

???

Yes, there's a Comments thingy; click it to read whatever comments are already there, and a field appears to add your own.

Unfortunately, the limit is 300 characters, and Yahoo refuses to give a counter in that one place it most needs it to show how many you've used or have left.

Just scroll down until you see the word Comments and click it.

You can add as many Comments as you want; it's unlikely anyone will see them, but one never knows.

Better to answer Open questions.

There is no universal button for all questions, you can add a comment individually to any question that is resolved.

You don't. Yahoo! Answers is a Q&A forum: you ask, you get answers, end of story. It's not a forum for conversation between askers and answerers. (And using Additional Details for this, unless it's for genuine clarification of the question, is likely to be a chat violation).