"Tau (uppercase Τ, lowercase τ; Greek: ταυ [taf]) is the 19th letter of the Greek alphabet. In the system of Greek numerals it has a value of 300. The name in English is pronounced /taʊ/ or /tɔː/ but in modern Greek it is [taf].
This is because the pronunciation of the combination of Greek letters αυ has changed from ancient to modern times from one of [au] to either [av] or [af], depending on what follows (see Greek orthography). Source: Wikipedia.
It has something to do with the confusion between "u" and "v" that was not distinct in Latin?
I had it rejected in the "type what you hear" exercise. I clearly heard "To Gramma Tav" and typed το γράμμα ταυ - but it was marked wrong and the "correct" answer was given as "το γράμμα τ." It seems unfair to mark the answer wrong when the directions are followed exactly. https://www.dropbox.com/s/nigmtstmmkd31r3/Screenshot%202018-04-01%2010.30.48.png?dl=0
Hi, try this website - you can spell the Greek out and cut and paste it into Duolingo for now: http://www.lexilogos.com/keyboard/greek_modern.htm
I just had the somewhat bizarre experience of two "type what you hear" questions with identical sound (το γράμμα ταυ) where one would only accept the single letter t, and the other only the spelled out version ταυ.
Not sure what is going wrong there, but wrong it is going.