"We are reading the letter about her grandma and your grandpa."

Translation: Čteme ten dopis o její babičce a tvém dědečkovi.

[psfinaki]

"O tvém dědečku" should be accepted, shouldn't it?

http://prirucka.ujc.cas.cz/?slovo=Dědeček

[VladaFu]

It sounds a bit weird. Czech national corpus has only 3 cases of "o dědečku" out of 5 billion database entries and all of them are names of some (fairy) tales. "O děděčkovi" has 270 entries.

"Po dědečkovi" has 1820 cases while "po dědečku" has 5. So on one hand it is not impossible, on the other hand it is not normal usage and can be confusing.

[LaynePierc1]

why do we need the demonstrative?

[VladaFu]

To specify it is the letter we already know about. Only sometimes you can do without it.

But here it is actually NOT required.

[AliceReiss]

Ve zvukovém záznamu paní říká: "velké o". To je matoucí, zvlášť když je "o" uprostred věty. Je to ve více větách.

[VladaFu]

Je to známá chyba a nedá se s ní nic dělat.