How do I create a sub-form for a form?
Using the form wizard
Just check the option "Add a sub-form" when it is offered. The wizard then asks for the field linking the form with the sub-form.
Without using the wizard
Open the form in design mode, either by selecting the form with a single click and then pressing the "Open form so you can change the selected layout" or by pressing the "Toggle design mode" button when the form is open for data entry,
Activate the form navigator to see the collection of controls. Right click on the name of the form, select "New > Form".
Give a name for the sub-form you're going to create, then the link between the table in the main form and the sub-form.