Would you describe yourself as a teacher? Should you describe yourself as a teacher? Could you describe yourself as a teacher?It seems to me that the question is not 'What's the difference between them' so much as 'Given that they are so very different, in what respects are they similar?'
b
As a matter of etymological interest, 'could' only has the same ending because a bunch of early prescriptivists thought it should look like the other modals; so they gave it a silent L. Chaucer, for example, used 'coude' (or maybe 'koude' - or maybe both [I haven't time to check]). ;-)They have the same ending? :-D