You can format the subform in single record view. Size everything and play with the format. You can end up what looks like nothing but a text box comment field. Play with the scroll bars, record selectors, and navigation button format.
Depends on how you want this to work.
Do you want to be able navigate back to the old comments?
Do you want to still see the comment after you enter?
Do you just want to hit enter and see just an empty text box?
If you set the property of the subform to data entry, then the subform always opens at a new blank record. Then you will need a button on the main form to move to another new record which in turns saves the existing comment
Private Sub Command12_Click()
With Me.CommentsSubform
.Form.Dirty = False
.SetFocus
End With
DoCmd.GoToRecord acActiveDataObject, , acNewRec
End Sub
You can also have two subforms. One in multi record view to show the list of existing comments, and the other in single form view for adding a new comment.