Modals And Semi-Modals?

Many sources call them 'semi-modals', Seyfihoca, though they may be grouped in various ways.

“Have to ... though semantically very close to must, has none of the modal properties and is clearly a catenative [verbs with verbal complements, like want to], not a modal.” (Huddleston, 1984: 165) But for others, such as Palmer, the meanings of have to and other forms including to be going to, to be able to, and would rather make them “semi-modals” (Palmer, 1990: 25).

The question of which syntactic role these semi-modals have in a sentence is problematic. Are the semi-modals auxiliaries or main verbs? Palmer does not directly address the question, but in a discussion on be bound to, he does make reference to the main verb being a verb of action, suggesting that he sees be bound to as an auxiliary (Palmer, 1990). Celce-Murcia & Larsen-Freeman (1983: 83) note that periphrastic modals (the semi-modals ending with to) “behave syntactically more like main verbs than do modals”, but a footnote at the bottom of the same page seems to suggest that they still consider periphrastic modals to be auxiliaries. They also note that “Structurally, have to is not truly a periphrastic modal since it requires do support .... In other words, have to looks like a verb and behaves very much like a periphrastic form in many contexts. Thus we have treated it like one.” (1983: 81).

Many of the semi-modals behave oddly. Used to often takes do support (Did you use to live in New York?), while need sometimes acts as a proper modal auxiliary (you needn’t come) and sometimes as a semi-modal requiring do (you don’t need to come). Had better shows the formal characteristics of modal verbs (no -s, no non-finite form, no chaining with other modals), but the presence of better makes treating it as a modal verb problematic, to say the least.

It appears that the semi-modals are hybrid forms, combining characteristics of both main verbs and auxiliary verbs. It also appears that the category is defined by the semantic functions of its members, not their formal qualities. This is important because it suggests that there is no necessary main verb or auxiliary verb characteristic that all semi-modals must share. In other words, students need to calibrate the individual structural characteristics of the semi-modals since each semi-modal has its own combination of main verb and auxiliary verb characteristics. They also need to learn when and how to substitute semi-modals for modal auxiliaries, and to be aware for the subtle changes of meaning these substitutions sometimes indicate.

See p. 2 here:

HUNTER COLLEGE READING/WRITING CENTER GRAMMAR AND MECHANICS The Verb System

Also:

The following verbs are often called "semi-modals" because they are
partly like modal helping verbs and partly like main verbs:

* need
* dare
* used to

Search this site (top right Search box) with:
need modal
dare modal
used to modal
and you'll find threads discussing them and examples, e.g.:

NEED ordinary verb/modal verb Any difference???
dare + verb infinitive form
Modal verb Used to, understood?