 Re: auxiliary
From a formal point of view, the true "defective" modals are 'can-could, may-might, must, ought, shall-should, will-would'. All of these may "take" all and any of the possible infinitives of a verb, i.e. active: base/simple, continuous, perfect simple, perfect continuous; passive: base/simple, perfect. Although 'need' and 'dare' are commonly thrown in with the "traditional" modals, not only may they occur as main verbs, but they suffer from certain restrictions.
'Need' is normally used interrogatively or negatively, but not affirmatively (except in a clause subordinated to some kind of negative, e.g. 'I don't see why he need bother', 'I doubt that he need apply''). Also, 'need' rarely (if ever?) takes a continuous infinitive, while 'dare' only takes the base/simple infinitive. Should we call them "partial" modals?
Structures like 'be (about) to, be going to, be able to, have to, be allowed to', etc., are often called "semi-modals" in EFL/ESL, as they complement and contrast in meaning/use/function with the traditional modals, and, as Dr Ibrahim rightly points out, some of them fill in for tense and infinitive forms that the traditional modals don't possess.  |