"He does not understand why."

Translation: Anh ấy không hiểu vì sao.

[atn173]

"Anh ấy không hiểu tại sao" should also be accepted, no?

[Moon_XO]

Yes, it would be accepted.

[Joesamhobbs]

Anh ấy không hiểu sao should also be accepted

[JosephLMac]

What in the world is wrong in using "tai sau" and not using "vi sau"?
Sorry no accents on this key board.
I am marked wrong!

[DoanMinhTho]

"Anh ấy chẳng hiểu sao." This sentence has the same meaning, does it not?