Halliday & Matthiessen (2004: 500-1):
The other part of the time-phase system, that has remained as a category of phase, is that of ‘duration/inception’: ‘durative’ going on, contrasting with ‘inceptive/conclusive’, starting and stopping. … Of these, the ‘go on’ term takes the imperfective; starting and stopping take either, with little difference in meaning — except that stop requires imperfective; stop + perfective is now interpreted as a hypotactic clause complex of purpose … . There is also an inceptive-durative ‘start to go on’ … .