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?

for example:

I ought to go now.

which role does to act there?

  1. 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?

  2. 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?

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    It's not required. I normally don't put the to in the phrase. – Phil Sweet Feb 9 at 12:07
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    Does this answer your question? have to / must / should / ought as applied to inanimate objects 'Ought (to) [and have to] are periphrastic modal auxiliary idioms' (@John Lawler). No doubt there was a reason why the odd string arose, but Crystal regards idioms as lexemes, the analysis of whose inner structure is rarely profitable. // Why does écouter not have an à, when English must transitivise using listen to ? – Edwin Ashworth Feb 9 at 12:13
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    1. is correct. The "to" is a subordinator, a special marker for VPs of infinitival clauses -- in your example "to go now", i.e. [I ought [to go now]]. Thus its category is 'subordinator' and its function is 'marker'. The same applies with your last two examples; The "to" is part of the VP heading the clause functioning as complement of "used": [I have [to go now]]" / "[I used [to smoke]]". – BillJ Feb 9 at 12:46
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    I find the idea of leaving out to really strange. I found this on ELL. – Kate Bunting Feb 9 at 12:49
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    @RamPillai Dare and need are a good example of the irregularities that come with modals. The to shows up only when they're not used as modals, in non-negative contexts. Similarly, negative ought not has an optional to, while affirmative ought requires it. Modals and negatives in the same sentence makes for very very complex and irregular grammar. – John Lawler Feb 9 at 17:18