_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)