In a main form / sub form scenario the main forms primary key resides in the
subform as a foreign key. This will relate all the subform data to the main
form. The main form should only show one record, the subform is the one that
should show multiple records.

For example I am a customer of XYZ company. When they call up me in the main
form, the subform will show all the orders I placed. Thus, the one to many
relationship.

If you are not getting the data you want, then either your table design(s)
is/are incorrect or your design of your mainform / subform is backwards.