There was no context other than the sentence itself which did not provide subject context to allow the selection of the subject that Duolingo chose. It seems to me that a rendering of the subject as either "I" or "he" or "she" should be correct. "I/he/she habia oido" should have been correct, it seems to me.
That's right. It'd be like this, aproximately:
"Había oído de este lugar" -> I had heard of this place. "Había oído HABLAR de este lugar." -> I had been told about this place.
I hope I've helped, and correct my mistakes, if need, because I'm not a native speaker ;)
Miguel [skype: miguel.alexandre36]
Yo había oído /I had heard. Yo he oído/I have heard. Tú has oído. You have heard. Tú habías oído/ You had heard. El ha/ había oído/ He had heard. Etc.
In English the twoSpanish forms are both translated by the past perfect, the same as
Yo canté y Yo cantaba are both rendered by I sang.
Sorry, but this could equally be translated as either He-She had heard about this place too: Yo había oído de este lugar. Él había oído de este lugar. Ella había oído de este lugar. Usted había oído de este lugar.
Había oído de este lugar. (sujeto tácito o implícito posible, fuera de contexto aclaratorio, yo, él, ella o usted).
If you use the tips or get a lot of answers wrong then Duolingo does not mark that skill as strong and it will decay quicker. Your tree will not stay golden as long. A great article that describes how it appears to work is http://gregreflects.blogspot.com/2014/12/how-to-make-gold-duolingo-tree.html