As to the relation between "guo1" and "guo2", it is argued that "guo1" denotes the completeness of the verbal action, and it can be found in the past, present and the future tense, while "guo2" only signifies the completeness of a past event.
"guo1" is the unmarked category, giving no specific information about the time when the verbal action takes place, while "guo2" is the marked category signifying the sense of "happened in the past".