try this:
acp -- forums/tag -- manage forums/on the left -- choose the forums you want to edit - click the edit icon [green one] -- ok
I hope this helps
pix below:
when i change a forum to category it just dissapears off board
you need to "Purge the cache" and set the permission
and you need to read this:
In phpBB3 there are no categories, everything is forum based. Each forum can have an unlimited number of sub-forums and you can determine whether each may be posted to or not (i.e. whether it acts like an old category). Here you can add, edit, delete, lock, unlock individual forums as well as set certain additional controls. If your posts and topics have got out of sync you can also resynchronise a forum.
so, the only way to seperate the categories/forums is:
creat a new forum choose "category" as a parent forum, then creat new forums choose "forum". that'll be ok