Click on New Posts in the menu bar that runs along the bottom of the box containing your User Name when you're logged in. That will bring up a list of unread posts - which could be for whatever length of time it's been since you were last logged in and reading. If you had read everything in a thread, or had marked everything in the Forum as having been read, that establishes the baseline from which the software
is "remembering" where you left off.
To mark everything read, click on Quick Links in the same menu bar, then select the second item from the drop-down list, Mark Forums
Read. That will zero out every post in every thread in every Forum, just as if you had read them all. Incidentally, the item just above Mark Forums
Read in the list is Today's Posts. It is the same as New Posts, but returns a list from the past 24 Hours.