Published online by Cambridge University Press: 02 June 2010
This article argues that be to is primarily a modal auxiliary expressing the necessity of future actualization of the ‘residue-situation’ (= the situation referred to by the clause minus be to). Eight possible ‘M-origins’ (= origins of the necessity) are identified. The ‘futurish’ use of be to in present-day English is closely related to these modal uses, especially to the use in which the M-origin is an official arrangement.
The modal interpretation shifts to a futurish interpretation when the emphasis shifts from the present existence of the necessity to the future actualization of the residue-situation. This shift of emphasis is accompanied by a loss of doubt about this future actualization.
In other words, the futurish reading comes to the fore when the (strong or weak) origin of the necessity is bleached, so that the hearer's attention is directed to the future actualization of the residue-situation. Various cases of such bleaching are treated. In some cases (e.g. when be to collocates with still or yet, as in He is still to keep the first of his promises), the bleaching of the M-origin is complete, so that only the sense of futurity (and hence of ‘not-yet-factuality’) is left. In some examples there is no clear difference between be to and will any more, so that the two are interchangeable within the same sentence.