Halliday & Matthiessen (2014: 568-9):
The difference in meaning between perfective and imperfective was referred to above. The general principle is that the perfective is ‘unreal’ and the imperfective is ‘real’; they may be opposed in any one of a number of contrasts, as future to present, appearance to reality, starting off to going on, goal to means, intention to action, or proposal to proposition; and sometimes the difference between them is minimal. The pairs of examples in Table 8-2 will give some feeling for the distinction.
