The Modern English progressive aspect is a periphrastic verb form composed of a conjugated form of the auxiliary be and the present participle of the main verb. At its origin in Late Old English, there were the two constructions illustrated by a and b.

Both constructions use the copula in a locating function. In a, its predicate complement is a prepositional phrase whose lexical head is an abstract noun derived by the suffix -unge from a verbal base. This construction has an absentive implication. The construction in #b has a present participle in the position of the predicate complement, thus already forming a periphrastic verb form.

The competition of two source constructions is the complicating factor in this case of grammaticalization. In Middle English, the construction using the verbal noun expands its use and gets desemanticized, forfeiting its absentive sense. is the formal successor to a.

As a consequence of the expansion and desemanticization, the participle ending -ende is ousted by the action-noun ending -ynge. At the next step, phonological erosion shortens both the preposition and the main verb form, while desemanticization deletes the locating sense ().

Further erosion deletes the preposition altogether, so the form hunting in this construction is no longer a verbal noun, but a non-finite verb form (b). At the same time, the erstwhile copula becomes an auxiliary, the new periphrastic verb form joins the conjugation paradigm and forms a binary opposition with the synthetic tense form (a).

The plain form of a has existed in the language since Proto-Indo-European times. The progressive form evolved at historical times, viz. in the Middle English period. It got entrenched in Modern English and became fully productive and regular. At present, it is ousting and renewing the plain verb form. Even in linguistic analyses we can read, in an appropriate context, the verb is agreeing with its subject instead of the verb agrees with its subject. The latter form is, of course, still possible. In this usage, the progressive form refers to a specific case, while the plain form is reserved for a general statement. Thus, the progressive aspect is further expanding and desemanticized.

References

Groot, Casper de 2007, ‘The king is on huntunge. On the relation between progressive and absentive in Old and Early Modern English’. Cultural Critique January 2007: 175-190 [online]