The "quote" button in any given post will automatically quote that post in a new reply on the same thread. Quoting anything else is a bit more manual. But if you just do "post reply" then the reply editor has a "quote" button and you can quote anything you like. Copy the text you want to quote, go to where you want to reply, "post reply", and then
Quote:
stick the new text into a quote box.
--paulr
