I just looked up 'about to' in both the new Cambridge grammar
(Huddleston & Pullum) and the Longman Comprehensive Grammar (QUIGLS).
QUIGLS classifies it under 'near-auxiliary' constructions, and note that
it expresses near future. It is similar to 'going to' in this respect.
H&P, on the other hand, list 'about' in a list of "adjectives
functioning as predicative complement and taking an infinitival
complement" (p.1257) through 'subject raising':

Carlos is about [Carlos wash his clothes] > Carlos is about to wash his clothes.
'Carlos' is raised to subject position of the 'upstairs' clause.

They list other adjectives that are in this category: 'able, fated,
liable, likely, set, sure, certain, due, wont'.

Constructions like 'be unable to', 'be apt to', 'be bound to' favor the
adjectival analysis; be willing to', 'be supposed to' , 'be able to' the
near-AUX analysis through their synonymy with pure modals like 'can', 'should'.



~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Johanna Rubba   Associate Professor, Linguistics 
English Department, California Polytechnic State University
One Grand Avenue  • San Luis Obispo, CA 93407 
Tel. (805)-756-2184  •  Fax: (805)-756-6374 • Dept. Phone.  756-2596
• E-mail: [log in to unmask] •      Home page: http://www.cla.calpoly.edu/~jrubba
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