Where is the best location for my primary keyword in my URL?

[TheEspresseo]

http://moz.com/learn/seo/url says:

http://www.example.com/category-keyword/subcategory-keyword/primary-keyword.html

However I am wondering about structuring things this a little backwards from that:

http://www.example.com/primary-keyword/ (this would be an introduction and overview of the topic described by the primary keyword)
http://www.example.com/primary-keyword/secondary/ (this would be a category landing page with snippets from articles within the niche described by the secondary keyword, which is itself a niche of the primary keyword)
http://www.example.com/primary-keyword/secondary/article-title/ (in-depth article on a topic within the scope of the secondary, which is within the scope of the primary)

Where http://www.example.com/primary-keyword/ is the most important page targeting the most important URL.

Thoughts?

[EricaMcGillivray]

Primary keyword in that URL example from the Learn SEO section refers not to the most important keyword on your site or the most important topic on your site, but to the most important keyword on that specific page that the URL is pointing to. I believe that's where the confusion lies. Hope this helps.

[aap82]

I think the best option is clearly:

http://www.example.com/primary-keyword/