Thankyou for that answer

Just to confirm, are you stating here that the perfective can be used to correspond to the english...

"Present perfect" - "I have eaten (now)." — "Я поел", "Я как раз поел", "Я только что поел".
"Past perfect" - "I had eaten, when he arrived" — "Я как раз поел, когда он пришел", "Я уже поел, когда он пришел".

AND also the "Simple Past" - for example in a story "The man ate his dinner and then went to sleep"??

Just want to make sure that's correct.

Note: just noticed it's funny (not funny) how your example "I had been eating for 5 minutes" has "HAD" in it and yet is imperfective, as you can tell by the fact the sentence doesn't show the action was completed and merely time was spent doing it, right (I had been eating for 5 minutes but didn't finish)? It seems you just need to learn to see things in binary imperfective/perfective rather than what the english grammar would lead you to believe. Are all sentences which include an element of time (spent) normally imperfective then?

Again, about the "I HAVE eaten before thing" I assume that is just also perfective with the added adverb for "before" stuck in their, to make it historical?

EDIT: I just read the proper terms are "resultative perfect" I have already eaten and "experiential perfect" I have been to america (sometime before). Any difference in Russian as in Icelandic or not?