When viewing a discussion, clicking “Chat” will make comments display from oldest to newest, so you can read them in the order they were posted, and clicking “Old” will do basically the same, but sorted by thread.
The “Communities” link at the top of the page takes you to a view that’s closest to the old-style forums front page.
If you click on Communities at the top of the page, and then click on Local, you’ll see the communities (similar to forums) hosted on this server.
And if you instead click on Communities and click All, you can view all of the different communities that this site is aware of. You can subscribe (for free) to any of them, and then the posts from that community will also be visible to you on this site(without having to go to any other sites). You’ll be able to respond to any of them straight from here, and you don’t have to log in again.
And if you click on Communities and then Subscribed, you can view only the communities you’ve subscribed to, regardless of whether they’re on this site or on other sites.
Lastly, if LemmyBB ever starts development again, I will look into adding it as an option(not removing the way things are right now, just giving the option for anyone who wants to use the old-school forums interface).