Halliday & Matthiessen (1999: 304):
As in English, the process is the category of experience that is located in time. In English, time is construed grammatically as tense: as a flow, with a more or less extended ‘present’ forming a moving but impermeable barrier between ‘past’ and ‘future’; and each instance of a process is located somewhere in the flow. In Chinese, time is construed grammatically as aspect: and specifically as an opposition of unfolding versus culminating; and each instance of a process may be given a value in this opposition, either ‘significant as unfolding — in its own right’ or as ‘significant as culminating — perhaps by virtue of its consequences’.