He said, " don't you understand what your father said? '' change into indirect speech

Indirect speech is when someone says what someone else said but not not in the same way

Direct speech: "Please look out for the bear!" she said.

So you could change it to something like:

he asked if I understood what my father said

Explanation: He told him/her that he didn't understand what his/her father said.

The reason reason for the his/her is because we do not know if he was addressing a female or a male in this case. When changing a direct speech to an indirect speech, we basically take the words spoken by the first party and become the middle man in relaying them to the third party. Since we don't know if the first party, whom the direct speech originates from was addressing a female or a male, it is safe to use both his/her to communicate the message to the third party hence converting it to an indirect speech.