Halliday & Matthiessen (1999: 215):
By comparison with temporal location, temporal perspective (aspect) is relatively backgrounded in English. (Some 20th century grammarians have interpreted the serial location as a kind of aspect, with "present in ..." as continuous, "past in ..." as perfect; but the earlier description as we have presented it here accounts more richly for the semantic patterning.) The temporal perspective takes over, however, when there is no deictic location (the clause is non-finite); in such cases, instead of making reference to 'now' the process is construed as either actualised, as in (on) reaching the gallery, turn left, or visualised, e.g. to reach the gallery, turn left. Sometimes the difference in meaning is very slight (e.g. a way of doing it/the way to do it)', but it is always there.