Halliday & Matthiessen (1999: 214-5):
Each of these variables differs from all the others; but at the same time, each is related to all the others, so that there are certain patterns of association among them. For example, a process that is unbounded (e.g. travel) is more likely than one that is bounded (e.g. arrive) to be put under temporal focus (e.g. while travelling is more likely than while arriving); a process located in the future is more likely to be beginning than ending (e.g. it will start warming up is more common than it will finish warming up). Some combinations may be more or less excluded: for example, a process that is beginning can vary in its perspective (e.g. the sun started to shine/started shining), whereas one that is ending is always actualised (e.g. the sun stopped shining; but not the sun stopped to shine). Thus, in any given language,
(i) one or other parameter may be given prominence,(ii) two or more parameters may be combined into a single semantic system,(iii) any parameter may be construed either more grammatically or more lexically, and(iv) a number of features that are not strictly temporal may be incorporated into the picture, both ideational ones like attempting/succeeding and interpersonal ones like the speaker's angle on the process — judgement of its likelihood, desirability, and so on.