2anta mu3allim mumtaaz yaa saam.
I have to say though, Mumtaaz is more like "excellent" and not "amazing." Amazing would be more of (Mud-hish) مدهش
Yá يا is the vocative article. Somehow equivalent to "O" in classical English. It is used before names to call a person and bring attention.
In Arabic orthography, the Kasrah or FatHah (as well as Dhammah of course) can be written with the shaddah accordingly ... either below Shadda if it Kasrah, or above Shaddah if it is FatHah or Dhammah. This said, however, some writers (calligraphers) might prefer to write Kasrah separately below the letter and keeping Shaddah on the top as it is. Just art.
I was confused because of this as well. It's actually not a fathah, it's a kasrah. If you look closely you can see that the line is bellow the shaddah. A fathah would be above the shaddah like this: اَّ
The audio for this one (as of August 2019) has no nunation. Is there a reason it would not be appropriate in this sentence? Or is Duo just being a bit random?