Subcategory Listings Page issues

Dear all,

I have read all the post about categories/subcategories/taxonomies but I cannot figure it out. I have 2 main categories with subcategories on my site:

Art
-18th century
-19th century
– modern
Books
-antiques
-modern

If a visitor adds a listing he can choose 1 category and I want it to be only a subcategory. However, it is also possible to select the main category but I only want the option of selecting a subcategory. I cannot figure it out how to do this. Who can help me with this?

Secondly, I want to make a navigationmenu where the 2 main categories are displayed with their subcategories in a dropdown list. If a visitor clicks on the subcategory from the dropdown menu, I want the visitor to see a page showing only those items of this subcategory. I tried creating a page for each subcategory and adding there [LISTINGS cat=’18th century’], [listings=’18th century], [categories name=’18th century’], [taxonomy name=’18th century]. However, without any luck. Who can help me with this?

Help would really be appreciated! Thanks in advance,

Kind regards,
Jelmer