Howdy,
I am trying to create a subform that functions the same as one in the Northwind Database that MS Access provides. The orders form has a subform and they are joined on the field OrderId. When you input data into the main form the same OrderID number is issued to the subform and underlying table. How do I get that to happen? How does the OrderID number get to the orderID table?

Study the relationships set up in the NWind db and see how the Primary key and foreign (link) keys are set up.

The foreign key in the 'many' table will automatically be entered when you add a record in the subform. This is set up in the Master and child links in the subform's properties.

hth