The Previous and Next buttons have two functions:

When viewing a forum index they scroll between the index pages (Next taking you to the next page of indexed threads and Previous taking yo back towards the opening page of indexed threads.

When viewing an individual thread the Previous and Next buttons currentky work in 'dateslip' mode (where, for example, the Next button takes you to the thread with the next most recent post).

Note if you view threads by creation date the action of the Previous and Next buttons may appear random because they scroll in order between latest posts, not by the creation date of threads.

Forum FAQ index.