Ought seems to have reached the end of an evolutionary phase in which it might have become a fully fledged modal. But the trappings of its older identity as a lexical verb have hung around - in the fact that to is almost always ther to link it with the following verb, and in the use of do support in negative statement. So while ought still works affirmatively as a marginal modal expressing obblogation, it's otherwise replaced by modals such as should and must in nonfiction writing of all kinds, everywhere in the world. (Pam Peters, The Cambridge Guide to English Usage, 2004)
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