What are themain differences betweenpast perfect and present per

What are themain differences betweenpast perfect and present perfect? Briefly explain them.
【正确答案】：The main difference betweenpast perfect and present perfect is: the present perfect is basically used to denote a relation between the past and the present, while the time frame of the past perfect is shifted from the present to the past, in which case it is typically used to refer to “a past in the past’’. Instead of the present moment being taken as the temporal point of orientation, a definite moment or period in the past is taken as the point of departure.