tutorial for inserting html into the middle of an existing page

Hello --

Can someone point me to a tutorial that shows how I can insert some
HTML tags into the middle of an existing page?

I have a survey form, and I would like to have the user give a comment
on a certain answer. They won't be commenting very often, but I don't
want pop-ups, and I don't want the comment textarea to appear after a
form submission.

I would like it so that if they click on a link, checkbox or button,
the page gets re-written to have some new textarea tags in the right
place, and then a textbox appears in the right place for the comment.

Is this doable, and is it a good idea?