"Your brother is writing a letter."
Translation:Twój brat pisze list.
there is no indication that this is plural "Your" and not the second person singular
Plural would be: Your brothers are writing a letter = Twoi bracia piszą list, no?
Accusative is a tricky thing for masculine singular nouns. When the noun is animate, the Accusative form has its form identical to Genitive. (with the exceptions of tata, mężczyzna and such).
When the noun is inanimate, the Accusative form is the same as the Nominative.
And then a lot of foods/beverages are grammatically animate because of absolutely no reason at all.
Since these wrong answers are automatically generated from words coming from other answers, I would take a guess there is an entry in the Polish course somewhere that looks like tat[a/o] and the system took only that part – that's why Knowledge base on Uservoice recommends writing such options like [tata/tato] instead. ;)
„Wasz tat” is meaningless.