[Robert B. Mercer]

Good question, and good luck explaining all this; it is not simple.

The general rule I go by is:

Use the present perfect continuous ( I have been writing textbooks)
instead of the present perfect (I have written textbooks) in examples of:

1. Unfinished activites

I have written textbooks implies you no longer write them,
I have been writing textbooks implies you still do

(in fact, the second sentence usually goes on to use FOR... I have been writing textbooks for five years.. the activity is certainly continuing.)

(One neat thing you can do by choosing one or the other, is to add that implication that some 'finished' thing might not be so finished after all:
I have lived in Oklahoma all my life.. pretty finished, pretty unchanging.
I have been living in Oklahoma all my life... well, it could be pretty finished...or... am I implying that I am about ready for a change?..."You know, dear, we've been living in this house for, what, thirty years now?" sounds like I'm implying that this is maybe an unfinished, about to change situation.)

2. Recently finished activities

It has been snowing this month... maybe it still is, but even if it is finished, it was recent. You wouldn't say It has been snowing last year, while you might say It has been snowing this year.


So if your student uses the "have been xxxing" for FINISHED activities ("I have been studying English for two years when I was a child") then it is incorrect.. it is (generally) reserved for ongoing activities that have largely occured in the past.

I teach present perfect first, for very completed activities, and then I introduce present perfect continuous for instances where the activity isn't quite perfectly finished....