"I do not know his surname."
Translation:Eu não sei o sobrenome dele.
You use "saber" about things you can learn, from books for example. "I know English/Eu sei Inglês" or "I know maths/Eu sei matemática" are some examples. You use "conhecer" about things you experienced, places you visited, or people you met.
For example, you can say: "Eu sei sobre Paris mas não conheço Paris". I know about Paris (because I read about it) but I don't know Paris (because I have never been there).
could you have said "o seu sobrenome"? Or would you still have had to say "dele"?
Oh thats good to know. I wish they hadnt given the wrong hint of apelido for surname though.
Perhaps because I know French, "Eu não sei o sobrenome dele" sounds wrong to me. I would say " Eu não sei qual é o seu sobrenome". Is the style of speech presented by Duo the standard? Does it sound natural to native speakers?