After reading a new forum topic you feel the urge to supplement, comment or argue on its content, you can do so by clicking the "Add new comment" link at the bottom-right of the topic.
On the page that appears, fill in the "Comment" field (it is required). You can also optionally supply a new subject in the "Subject" field. You cannot attach files to a comment. You can, however, add click-able links to your comment. To do so, select the text that has to be turned into a link and click the second button from the left on the button bar above the "Comment" field. Fill in the "Link URL" and the "Link title" fields and click "OK".
When finished, click "Preview" on the bottom of the page to have a preview of your comment. When satisfied with the outcome, click "Save" to actually post your comment on the forum.