Ability/Potentiality

Halliday & Matthiessen (2004: 621):

This is on the fringe of the modality system.  It has the different orientations of subjective (implicit only) realised by can/can’t, objective implicit by be able to, and objective explicit by it is possible (for …) to.  In the last of these, the typical meaning is ‘potentiality’… .  In the subjective it is closer to inclination; we could recognise a general category of ‘readiness’, having ‘inclination’ and ‘ability’ as subcategories at one end of the scale… .  In any case can in this sense is untypical of the modal operators: it is the only case where the oblique form functions as a simple past…