>However, I meant putting two modals together:
>* you should can
>* he will must
>* you ought to may
Of course, you are right about that, but we do find examples of such occurrences in substandard English. For example, in some novels by Barry Gifford (of 'Sailor and Lula' fame), you find dialogues containing things like:
'Might could she will!'
(Yes, I'm aware that 'might could' is just another way of saying 'maybe')
> I don't see why it would be much simpler
From a teacher's point of view, it's simpler to teach a rule that has NO exception, instead of performing a sort of linguistic contorsion that forces you to say: 'OUGHT is a modal, BUT it doesn't work like the other modals.' If you put OUGHT (to do) in the same group as HAVE (to do), BE ABLE (to do), BE ALLOWED (to do), NEED (to do), things are definitely much simpler.
> It is really true that no two modal sentences can be synonymous if the verb is different? Hmm, possibly, but, then again, possibly not.
OK, I'd love to have some examples of that. Again, 'You might be right' does not mean the same as 'You may be right'.