Form Authentication with Active Directory member role...

http://blogs.msdn.com/b/sridhara/archive/2010/01/07/setting-up-fba-claims-in-sharepoint-2010-with-active-directory-membership-provider.aspx

http://blogs.technet.com/b/speschka/archive/2009/11/05/configuring-forms-based-authentication-in-sharepoint-2010.aspx

http://technet.microsoft.com/en-us/library/ee806890.aspx#section2

We followed as per sridhara site. Everything worked fine.

After doing this, while login to site with form authentication we got error like below

The server was unable to process the request due to an internal error. For more information about the error, either turn on IncludeExceptionDetailInFaults (either from ServiceBehaviorAttribute or from the configuration behavior) on the server in order to send the exception information back to the client, or turn on tracing as per the Microsoft .NET Framework 3.0 SDK documentation and inspect the server trace logs.

Followed this link and solved our problem...
http://sharepoint-2010-world.blogspot.com/2011/03/server-was-unable-to-process-request.html