A modal verb (also modal, modal auxiliary verb, modal auxiliary) is a type of auxiliary verb that is used to indicate modality – that is, likelihood, ability, permission, and obligation. Examples include the English verbs can, must and should.
In English and other Germanic languages, modal verbs are often distinguished as a class based on certain grammatical properties.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 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. 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.
