The Present Day English construction to be about to, a SEMI-AUXILLIARY which locates the event frame of the main verb in the immediate future, exhibits some unique characteristics in aspect and modality when it occurs in the negative or in the past tense (future-in-the-past). These specific semantic properties appear to be rather difficult to account for in a strictly synchronic framework. An investigation into the diachronic sources of this construction in the framework of grammaticalization theory reveals that these divergent properties are actually vestiges of the original structures from which the Present Day English form grammaticalized. Thus, an account of the diachronic origin of to be about to is appealing for both its explanatory and its theoretical significance: first, it offers a unified explanation of the semantics of the Present Day English form and, second, it demonstrates the application of diachronic analysis to the explanation of the properties of synchronic structures.
"Synchronic Applications for Diachronic Syntax: The Grammaticalization of to be about to in English,"
Colorado Research in Linguistics: Vol. 15.
Available at: https://scholar.colorado.edu/cril/vol15/iss1/3