A modal verb is a type of verb that is used to indicate modality – that is: likelihood, ability, permission, request, capacity, suggestions, order, obligation, or advice. Modal verbs always accompany the base (infinitive) form of another verb having semantic content. In English, the modal verbs commonly used are can, could, must, should, had better, have to and sometimes need or dare. In English and other Germanic languages, modal verbs are often distinguished as a class based on certain grammatical properties.
A modal auxiliary verb gives information about the function of the main verb that it governs. Modals have a wide variety of communicative functions, but these functions can generally be related to a scale ranging from possibility ("may") to necessity ("must"), in terms of one of the following types of modality:
epistemic modality, concerned with the theoretical possibility of propositions being true or not true(including likelihood and certainty)
deontic modality, concerned with possibility and necessity in terms of freedom to act (including permission and duty)
dynamic modality, which may be distinguished from deontic modality in that, with dynamic modality, the conditioning factors are internal – the subject's own ability or willingness to act
The following sentences illustrate epistemic and deontic uses of the English modal verb must:
epistemic: You must be starving. ("It is necessarily the case that you are starving.")
deontic: You must leave now. ("You are required to leave now.")
An ambiguous case is You must speak Spanish. The primary meaning would be the deontic meaning ("You are required to speak Spanish.") but this may be intended epistemically ("It is surely the case that you speak Spanish.") Epistemic modals can be analyzed as raising verbs, while deontic modals can be analyzed as control verbs.
Epistemic usages of modals tend to develop from deontic usages. For example, the inferred certainty sense of English must developed after the strong obligation sense; the probabilistic sense of should developed after the weak obligation sense; and the possibility senses of may and can developed later than the permission or ability sense. Two typical sequences of evolution of modal meanings are:
internal mental ability → internal ability → root possibility (internal or external ability) → permission and epistemic possibility
obligation → probability
The verbs/expressions dare, ought to, had better, and need not behave like modal auxiliaries to a large extent, although they are not productive (in linguistics, the extent commonly or frequently used) in the role to the same extent as those listed here. Furthermore, there are numerous other verbs that can be viewed as modal verbs insofar as they clearly express modality in the same way that the verbs in this list do, e.g. appear, have to, seem etc. In the strict sense, though, these other verbs do not qualify as modal verbs in English because they do not allow subject-auxiliary inversion, nor do they allow negation with not. Verbs such as be able to and be about to allow subject-auxiliary inversion and do not require do support in negatives but these are rarely classified as modal verbs because they inflect and are a modal construction involving the verb to be which itself is not a modal verb. If, however, one defines modal verb entirely in terms of meaning contribution, then these other verbs would also be modals and so the list here would have to be greatly expanded.