When you first arrive at the forum page (after clicking on the "discussion forum" link in one of the different language menus), you will see a list of interventions on the forum. It will look something like this:
How to intervene in the forum
If you want to post a new intervention yourself, then click on the link indicated by "2" above (Show Post Form) and you will see this:
How to read visitors' interventions
If you click on the link indicated by "1", then you will open that intervention to see what comments this visitor has made on the site.
How to comment on a previous intervention
If you click on the link indicated by "1", then you will open that intervention to see what comments this visitor has made on the site.