HelloWorld1

trigger 
HelloWorld1
on
 Account
 (before delete, after delete, after undelete) { 
       if(Trigger.isBefore)
        {
          if(Trigger.isDelete)
          {
             if(PClass.firstRun)
            {
                 Trigger.old[0].addError('Before Account Delete Error');
                  PClass.firstRun=false;
              } 

           }

        }

}