In English, we have both the present perfect and the present perfect progressive (or continuous), and they often imply different things. Here are examples illustrating the two tenses, respectively:
1) I have written the letter.
2) I have been writing the letter.
In 1), we know that the letter has been written. We don't know when it was completed, but we do know that it's all finished. In 2), the situation is much more ambiguous. The writing has continued up to the present, or at least very close to the present. But we don't know whether the letter has been finished, and the implication is actually that it has not been finished.
Anyway, does Greek have any way of differentiating between 1) and 2)? Obviously, 1) would be:
Έχω γράψει το γράμμα, or
Έγραψα το γράμμα.
But what about 2)? Is there any way to convey the progressive perfect meaning in Greek?