A subform is a form within a form. Subforms are especially useful when you want to show data from tables or queries with a one—to—many relationship. For example, a Customer form might have a subform that displays each customer's Orders.
The main form and subform are linked so that the subform displays only records that are related to the current record in the main form. For example, when the main form displays a particular customer, the subform displays only orders for that customer.
It's very important that the underlying tables or queries of the main form and subform have a related field and that the related field appears somewhere on both forms.
Now you need to select the fields you want to appear in the subform. You must select the related field used to join the main form and subform. This related field must appear on both the main form (where it is called the parent field) and on the subform (where it is called the child field).
The next step in the Wizard is to define the fields that link the main form and the subform. The Subform Wizard is often smart enough to recognize the field and use it to link the two forms. If not, you will have to click the Define my own option and select the two related fields.