I currently have an existing asp.net website with default security/membership. i want to add yaf forums but in a way that current members of my site can log in to forums as well using their former credentials.

how is this thing possible?

how can i link yaf with my existing users table when yaf has it's unique database structure regarding membership and userprofile?

and can I run yaf in a sub domain but still integrate it with my current membership system or am i forced to run it in a folder within my root to be able to use integrating features?

There is a documentation on how to Integrate YAF in your Application here...

https://github.com/YAFNET/YAFNET/wiki/YAF.NET-Integration-in-to-an-existing-ASP.NET-Application 

Regarding the existing Users and how to import them in to YAF, under Admin -> Users & Roles -> Users, it will show all users that are in the Membership but not in YAF. All you need to do is press the Sync Membership User Button to import the users in to YAF.

my website http://aqarspot.com  is in Arabic language do i have to make other steps