When I come upon this sentence: I have to do something, I thought have to of it acts like a phrase, where the meaning is having an obligation. And have to is an auxiliary verb like must. But after bring its question and negative sentence: do I have to do something?, I don’t have to do something, something was wrong, I thought, and looked up CGEL. They say have is not an auxiliary but a catenative verb that takes non-finite complement.
From all this, I get the question: which one, have or to, has the meaning of the obligation? From OLD (to as an infinitive marker,#7), to has the obligation meaning when it gets along with be verb. Then is it still valid that in have to to has the obligation meaning?