The general principle is that the perfective is ʻunrealʼ and the imperfective is ʻrealʼ; 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.
- Halliday (1985), Pag. 256