There is no Notes & Tips section for this subject
There are, but I see that they're not accessible to learners from the tree.
I wonder whether it's due to the colon in the URL; I've tried to remove it and I wonder whether that will make them available in due course.
The perfect tenses are formed from a form of the verb έχω (present tense for present perfect, past tense for past perfect) and what's called the απαρέμφατο "infinitive" in Greek grammars -- as far as I know, it's always identical to the third person singular of "aorist subjunctive" (the form you would use for the simple future).
For example, "he will read" would be θα διαβάσει, and so "I have read" is έχω διαβάσει and "you had read" would be είχες διαβάσει.
πηγαίνει would be present subjunctive, not aorist subjunctive -- θα πηγαίνει would be "he will be going" (regularly) rather than "he will go (once)" which is θα πάει. Hence "they have gone" is έχουν πάει.
That might be because the skills for Present Perfect and Past Perfect have... moved around a bit in the new tree, so the tips added to the new tree didn't get automatically added to the old tree. However, they had just been added. Please, make sure to check back in a few hours, and let us know if they are visible. The T&N take a while to update. ^.^
'Been' means 'πάει' only when referring to having been to a place. In Greek, you can be (είμαι) somewhere, or you were (ήμουν) some place, but it is not possible to have been there. 'Have gone' = 'έχω πάει' is the only equivalent.
Many verbs in Greek do not form all the tenses. To be/είμαι is one of them.
Just so you're aware, I just wrote the answer as "αυτοι εχουν φαει ολλοι μαζι" from the listening exercise and it was marked correct! I quite often find that when I make small mistakes (which here add up to a totally different meaning) it marks it as correct, when it would be more helpful to have the typos pointed out, even if you're generously giving me a green. This is especially true when I'm trying to remember if the damn word is spelt with a υ η or i, for example, I'll get marked as correct and maybe think "Ok, try and remember it's φαγιτο!!" and then a few questions later I'll see it should have been φαγητο (if I'm getting that right this time...). Other times I get green and the typos marked for me. Wondering what the reason is for not always doing that.
When your sentence is graded you will see at the bottom of the page "Report" with this list.
-My answer should not be accepted.
-The English sentence is unnatural or has an error.
-One or more of the options is a duplicate.
-One or more of the options is offensive.
-Something else went wrong.
The top one is to alert Duolingo that the sentence should not have been accepted and they will need to correct the system. As you can see Duolingo is aware that anomalies occur and give the learner the chance to report them.
Or you can report it here,