Thanks.. looking through those.
Seems though that the explanations there present a similar idea to the use of the two tenses as in English (i.e. time dependent - finished or ongoing action).
But in a case like this one (in British English at least; American is somewhat different): "Have you finished?", the Present Perfect tense is used for an action not ongoing but recently completed. "I've had an accident" would be another example of that, (the listener understanding that the results of the past (and finished!) accident are visible now, at the present time. If one had said "I had an accident", 'αόριστος', the listener's probably confused reaction would be to wonder when. Years ago?). In Greek though, I would be surprised to hear 'έχεις τελειώσει?', expecting instead 'τελείωσες?' (though to my ear both are acceptable. Is there a difference, subtle or otherwise, in meaning though?). In British English at least, the Simple Past, "Did you finish?", is wrong there of course.
Or, where no time period is implied, such as "I haven't seen him", the Greek would surely more likely to be 'Δεν τον είδα' and not 'Δεν τον έχω δει'. Or is that not so? Maybe the αόριστος tense is just more colloquial?
Problem is, I find myself wanting to use the παρακείμενος in Greek exactly where I would use the Present Perfect in British English, and the αόριστος where I would use the Simple Past. But that can't be right... (In the end, I rarely use the παρακείμενος tense at all in conversation, preferring most times the αόριστος, which I think (?) is what I used to hear when I lived in Crete).
I'll continue looking through the links you posted meanwhile. Thanks again.