The the OnExit event of the last control on Main form use this code:
Me.NameOfSubFormControl.SetFocus
Me.NameOfSubFormControl.Form!NameOfFirstControlOnSub.SetFocus
Be sure that you replace
NameOfSubFormControl
with the actual name of the subform ***control*** which is not necessarily
the same as the name of the form being used as the basis for the subform.
As you can see, you have to set focus to the subform control ***then*** set
focus to the textbox on the subform.