I am having trouble with my left navigation. Basically, I have several top level pages and then sub pages underneath these pages. I want the sub-pages to display in the left navigation only when I am on the corresponding top level page or any of the sub-pages within that section. Is this possible?
I believe you want to use <% if LinkOrSection = section %>
LinkOrSection will return section if you are on that page or a child of it and link otherwise. See the documentation for more information - http://doc.silverstripe.org/built-in-page-controls#links_and_classes