You don't say HOW you are opening the input form, but you could have 2 buttons that would open the form in different modes:
One button ("btnAdd"), would have the following code that opens the form in "ADD" mode:
Code:
Private Sub btnAdd_Click()
'The user can add new records but can't edit existing records.
DoCmd.OpenForm "InputFormName", , , , acFormAdd
End Sub
The other button ("btnEdit"), would open the form in EDIT/Add/Delete mode:
Code:
Private Sub btnEdit_Click()
'The user can edit existing records and add/delete records.
DoCmd.OpenForm "InputFormName", , , , acFormEdit
End Sub
Change "InputFormName" to the actual form name.