I don't think duo is the best way to inforce your language, cause it only gives you basics, i know it from my experience, i mean my mother tongue is Arabic and english is my 3rd language after french, both arabic and french are just basics. Languages here are to let you communicate with others, but not for letting you going deep into the language.
Your question can be answered by using the facilities provided in the course. The audio here is very clear. The first skill has an analytical explanation of the alphabet and its pronunciation see Tips&Hints at the top left. A glance at the Koine Greek pronunciation here will show that the Ππ is pronounced the same as in the modern Greek taught here. Hope that helps.
In addition, you may want to look at the Forums: here .
Yes. If it ends in a stressed iota, it will move. If not, it won't.
However, if it ends in an unstressed iota, it will move to the end in the genitive case, e.g. το σπίτι - του σπιτιού - τα σπίτια - των σπιτιών.
(Certain other single-syllable words also have a stressed genitive case in the singular and/or plural if those forms have an additional syllable, e.g. το φώς - του φωτός.)
This is explained in the Tips & notes for Basic 1. At the start of each lesson you 'll see a small light bulb image click on that and you'll see a lot of information. All Skills have these. Here is just a small part of the Tips & notes from Basic 1.
Examples: To BE
(Εγώ) είμαι = I am
(Εσύ) είσαι = you are (singular)
(Αυτός, αυτή, αυτό) είναι = he, she or it is
(Εμείς) είμαστε = we are
(Εσείς) είσαστε = you are(plural) (or είστε)
(Αυτοί, αυτές, αυτά) είναι =they are
Important note: the pronoun (Εγώ, εσύ ...is not always needed.)