Form not carrying names to table behind the subform.

I have a subform linking to a tbl_class. The tbl_class has an ID, LastName,
FirstName, and ClassDate. ClassDate is date completed. (Access 2003)
The relationship is one on the main form to many on the subform. The ID is
populated from the main form and the ClassDate is entered.
However, the LastName and FirstName does not carry over from the main form.
I must be doing something wrong?
People leave the organization every three years or so. They take many
classes and their names should remain in the tbl_class for historical and
validation purpose.
How can I get the LastName and FirstName to populate the tbl_class after I
have entered the ClassDate but before I move onto the next record?
Appreciate any help!
--
Gil Silva