"저는 제 선생님 성함을 몰라."
Regarding the sentence : "저는 제 선생님 성함을 몰라요." from the Honorific skill, would 선생님의 be correct if it was used here?
In addition, could "I do not know the name of my teacher" be a valid translation? It was not accepted, and the answer provided is "I do not know my teacher's name" , so I am wondering if, in that translation, a subtle part of the original sentence's meaning was lost (I've noticed Korean particles can sometimes subtly modify a sentence's meaning).
'의' is just a marker which means "'s". It has nothing to do with Honorifics.
In the native Korean language, the "noun1 noun2" is perfectly natural. In English, we say "country name", not "country's name". We could say the latter, but the first one is more natural. Same goes for the Korean language.
친구 이름 - more natural 친구의 이름 - grammatically correct, but not necessary.
선생님 성함 - more natural 선생님의 성함 - grammatically correct, but not natural
The most natural expression is: "저는 우리 선생님 성함을 몰라요."
Use 우리, not 제.
If you really want to be appropriate and use honorifics, you have to change the expression itself. "저는 우리 선생님 성함이 어떻게 되시는지 몰라요."
Yes, to the second question.
Duolingo's Korean is very poorly made. It has weird, unnatural expressions. When you use Duolingo, you have to give what the machine translation knows, not actual, natural Korean expressions.
Thank you so much for taking your time to write such a detailed explanation. I am just in a beginner level, so every resource I had insisted in using 의. I will keep what you said in mind. It was very helpful.
I also noticed that Duolingo is strict with translations, but I thought the reason behind it was that it is really difficult to translate these languages into each other. Nevertheless, it helps me with word order, with which I struggle a lot.