Grammaticize

the very first attempt to embody, to arrange, or to grammaticize, this language

... organization (hence its heavy reliance on passives), but does not seem to grammaticize any person-based constraint. In languages with canonical inverse constructions (say Algonquian languages), the non-local person-based constraint is the primary determinant of argument encoding, and ordinary notion of topicality (which is local) plays little role.

Languages differ in the extent to which they grammaticize forms within this constrained inventory of conceptual domains and individual concepts, and this inventory, Talmy argues, amounts to the fundamental conceptual structuring system […]

Or to put it another way, the present tense use did not grammaticize. I suggest that the reason for this is that the habitual was more frequently used in the past than the present, since in the present there would be fewer cases […]

Clause initial position is a frequent place for anaphoric elements and markers of textual coherence to grammaticize. The grammaticalization of a QP from a disjunction with anaphoric function fits well into this picture.

Thus it appears that one avenue by which cunnan begins to grammaticize as an auxiliary is determined by the fact that it was already frequent and had already undergone some weakening of its semantic content.