set serveroutput on
create or replace trigger Before_Insert_Trigger
before insert on teacher
begin
       dbms_output.put_line(:new.f_name);
end;

create or replace trigger Before_Insert_Trigger
                          *
ERROR at line 1:
ORA-04082: NEW or OLD references not allowed in table level triggers

ORA-04082: NEW or OLD references not allowed in table level triggers 
Cause: The trigger is accessing "new" or "old" values in a table trigger.
 
Action: Remove any new or old references.