Are modal verbs adverbs?

Morphologically, a modal verb is nothing more than a verb since it carries time, mood and person. However, from a functionalist perspective, we observe that the content of a modal verb, like that of an adverb, affects the relation of predication. This behavior is clearly not that of a verb, but rather that of an adverb. This somewhat iconoclastic view is made possible by the functionalist model, as well as by Tesnière’s concept of transfer (translation), which posits that words cannot be reduced to morphological entities. Moreover, this conception echoes an intuition of G. Guillaume, who defines the adverb as an incidence to an incidence.