Halliday & Matthiessen (1999: 517-8):
Processes take place in space-time, which the grammar may model in a variety of different ways: the happening is upstream or downstream, past or future, real or imaginary. Typically the point of reference is the speech situation: there is some deictic feature relating what is being said to the current "moment" in time and space. In English the deixis is achieved by locating the process on a linear time-scale with 'present' as a fulcrum between 'past' and 'future', or else by locating it on one or other of a cluster of scales whereby the speaker intrudes his or her own judgment on it. There may be a wide variety of other attachments to the verb — modalities, aspects, phases and the like, which the grammar construes as features of the process; consider English examples like wasn't going to start trying to help. On the other hand, the processes themselves are not, in general, construed into systematic taxonomies, and the verb is expanded by grammatical rather than lexical means.