how to add subforum for 1 section only ?

[PAKIDIL]

Hello,
i would like to know how to make subforum for only section ?

example :

Forum category : Collection
Forum : Big Collection (visitor click on big collection that leads him to the list of sub forums)
subforum>A-collection
subforum> B - collection

and so on.

hope you guyz get my point . waiting for respose.
thanks
best regards