Translation:Žofie did not open the letter from Kateřina.
I would guess because "Kateřina's letter" could mean either a letter FROM Kateřina, or a letter from someone else TO Kateřina, while the original Czech sentence explicitly refers to a letter FROM Kateřina. (Maybe one of the moderators can give you a more authoritative answer.)
"Zofie did not open a letter from Katerina" Is "the letter" the only possibility? To me both seem correct.
Here we speak about some concrete letter from Kateřina, so "the" is the only possibility.
That is quite extraneous and stresses it more I would translate it as "that letter". In Czech the determiner is nit required, "od Kateřiny" specifies it enough, while the English grammar requires "the" here.
It is similar to "the first man" = "první muž (or člověk)" and "ten první muž" would be "that first man"