Hi Muhammad Saqib,
You have the parent ID on the page Url because you have set SingleForumsPage url property as described in the previous post. So, the parent ID is added as a QueryString in the url. The Forums module implementation expects the ParentID in the url because we use this guid internally and without it the module will not work. The forums on Sitefinity.com uses completely different implementation.
However we will change the forums module behavior from Sitefinity 4.0 and we will use the name of the articles in the url, currently this is not possible.
All the best,
the Telerik team