Trigger compilation problem

Hi, I have written an after insert update trigger. However, while running it is giving error.
I am new to trigger. Could someone give me advise how to handle the errors here. Thanks
in advance. Regards.

SQL> CREATE TRIGGER emp_a_iurow

   after insert or update of employee_id, last_name, first_name, middle_initial,

   dept_id, title, supervisor_id on employee    for each row
   begin
   if inserting then
   insert into employee_log(employee_id, mod_user, mod_timestamp)    values(:new.employee_id, username, timestamp);      else
    insert into employee_log
     values(:old.employee_id, :old.last_name, :old.first_name, :old.middle_initial,

    :old.title, old,supervisor_id, username, timestamp);    end if;
   end;

  /

Warning: Trigger created with compilation errors.

SQL> show error;
Errors for TRIGGER EMP_A_IUROW:

LINE/COL ERROR

-------- -----------------------------------------------------------------
3/1      PL/SQL: SQL Statement ignored
4/26     PLS-00201: identifier 'USERNAME' must be declared
6/1      PL/SQL: SQL Statement ignored
7/93     PLS-00201: identifier 'OLD' must be declared