αυ/αύ is "af" and ευ/εύ is "ef" when: they are followed by a voiceless consonant θ, κ, ξ, π, σ, τ, φ, χ, ψ or nothing e.g. αυτό [afto] (this), εύκολος [efkolos] (easy), ταυ [taf] (tau - the 19th letter of the Greek alphabet), ευ [ef] (well [archaic]).
Otherwise av, ev, that is before vocals and sounding consonants: αύριο [άvrιο] (tomorrow), Ευρώπη [Evrόpι] (Europe)
Τ, τ has the name ταυ in Greek which is pronounced taff in modern Greek and tau in old Greek. In English the scientists gave names to the Greek letters transcribing old Greek and pronounced them accordingly. The scientific system is still widely used.
Here you can listen to the alphabet both with modern pronounciation the old (Erasmus) pron. https://www.omniglot.com/writing/greek.htm and a lot of other interesting things
I'm sorry this skill has some sort of bug. My advice since you know the Greek letters well is to high tail it out of there. If you want to review the letters go here: https://forum.duolingo.com/comment/23430853 there is even a source to listen to the pronunciation of each letter.
From your profile, I see you've completed nearly everything. If you want more practice you could go back to .... Abstract Objects 2 where there is a tiny bit you can review.