This chapter summarizes data about the modals and their ancestors from Old English and Middle English, in order to provide a foundation for Chapter 3, which discusses various proposed explanations for their history and behavior. All example sentences, unless otherwise indicated, are from Visser.
Warner (1993) describes the traditional criteria for distinguishing auxiliaries, including the modals (mainly can, could, may, might, must, shall, should, will, and would). These are the so-called NICE properties: negation, inversion, clisis (or coding), and ellipsis (or emphasis). The not of sentence negation follows the auxiliary (modal or do), but never a full verb; the auxiliary and subject invert in various interrogatives, unlike full verbs; some auxiliaries may have reduced clitic forms; they can also occur as a single element in a sentence and encode the meaning, as in the following sentence:
(1) He must.
They may bear sentential stress and may appear in elliptical constructions without their normal complement, where the sense is to be retrieved from the linguistic context. Warner calls the last "post-auxiliary ellipsis" because it is dependent upon the auxiliary. Auxiliaries also fail to co-occur with periphrastic do.
The present day modals in general are subject to the above criteria. They also have several further distinguishing features: they lack nonfinite forms in the standard dialect; they lack present indicative 3rd person singular agreement; the "core" modals are followed by a plain infinitive; and the relation between the present and past tense forms is seldom simply temporal. At earlier stages of the language, however, the ancestors of today's modals were not as distinct from other verbs as they now are, either in terms of their syntactic behavior or their morphology. I shall use Lightfoot's term "pre-modal" in the sense of "ancestor of a present day modal" for the sake of convenience.