Say for example, the subform name = SubForm
and the subforms Record Source = SubFormQuery
Insert the following VBA code using the Main Forms On Open Event:
If DCount("*","SubFormQuery")=0 Then
Me.SubForm.Visible = False
Else
Me.SubForm.Visible =True
End If
That way if the subform is not empty, it will be visible and you can enter data into it. If it is empty, it will be invisible. Substitute the subforms actual Record Source for 'SubformQuery' and the subforms actual name for 'SubForm'.