That's actually not easy to do.  In a bound Form / Subform combination,
there is an feature called "AutoSave" running in the background.  When the
Focus moves from the main Form to the Subform, the Record bound to the main
Form is automatically saved.  Similarly, the Record in the Subform is saved
when you move the Focus from the Subform to the main Form.

You cannot disable this AutoSave feature.  Hence, what you wanted cannot be
achieved through the normal use of the Form/Subform combination.

There are some work-arounds but they are fairly messy and involve a fair bit
of coding.

You may have to think of another strategy for data entry such as hiding all
the Subforms while the user enters the Record on the main Form and force
them to explicitly save this Record before showing the Subforms.

Alternatively, you can just create a normal Form for the "One" Record (in
the old main Form) and CommandButtons to go to separate Forms to enter the
"Many" Records (fr the old Subforms).