Halliday & Matthiessen (1999: 214, 215):
In transforming experience of time into meaning, human communities have evolved a number of basic parameters. We can identify four of these that are relevant in the present context:
(1) the temporal staging of a process: it may be beginning, taking place or ending. underlying concept: a process occupies a certain measure of time … [grammaticised as phase] …
(2) the temporal perspective on a process: we may frame it in or out of temporal focus. […] underlying concept: a process relates to the flow of experience as a whole, including other processes … [grammaticised as aspect] …
(3) the temporal profile of a process: it is either unbounded or bounded. underlying concept: a process has the potential for being extended in time … [grammaticised as aktionsart] …
(4) the temporal location of a process: it can be related to 'now' as past, present or future. underlying concept: a process takes place within a linear flow or current of time … [grammaticised as tense] …