I've seen many explanations that just treat ought to as a whole which equals should. but how to explain the function/role of each word separately?
I ought to go now.
which role does to act there?
where to is just part of the to-infinitive to go (as a mere infinitive marker, no meaning) if so, why to-infinitive marker is required after ought, but not after should?
to is part of the phrase ought to, if so what's its function/role at all?
I have similar questions in understanding auxiliaries like have to and used to also, so how to understand this kind of structure correctly?