Duolingo is usually obsessed with "She" when there is no predetermined context, though usually accepts "He" or "I" or "It". However, here they have decieded to go with "I" and reject "He".
Is this just craziness or is there something in this sentence that means it has to be "I" and not "He"?
I've always known the saying as "The word on this street is..." - i.e. a different placement of the verb. But of course this reflects (basically) what Duolingo already offered above - though Duo's sentence is in a rather uncommon form; if I encountered this exact wording, I'd have to think twice before I understood what was intended.
Well normally it's a different verb form for I pronouns, as opposed to you pronouns or he/she/it pronouns, but with past perfect they are all the same, so in this sentence, how do you know if "habia oido" is supposed to mean "I had heard," as opposed to "you had heard," or "he/she/it had heard"?
Ah, I understand. You're right - it's like that with the imperfect past tense as well.
In Duo, you just have to guess which one you think it might be and hope that it's right! There are plenty of examples where they haven't added a certain response to their database of correct answers (for example, accepted ella or él but not usted) and if that happens you have to use the report function to notify them of the problem.
In real life, context will usually take care of any ambiguity, but in Duo it's really just a crapshoot.
Hmm, I don't think it quite works like that, even though there is no pronoun, 'cosas' cannot be the subject either because it is the object in this instance. Otherwise it would be more "cosas habían sido oír en la calle". As mentioned in a previous comment, pronouns are usually used with past perfect sentences, but because Duolingo isn't a person it has not used pronouns probably because it may not be easy to fix.
Not necessarily. There is an assumption that the subject is known through some greater context, which is absent here. It's similar to wondering who "he" or "she" is. I mean, suppose we explicitly included the subject pronoun "ella." How do you know who she is? It could be Mary, or Jane, or Janelle, or .... And even if we knew it was Mary, how do we know if it's Mary Shelley or Mary Stuart or Mary Tyler Moore or ....
The point is that we understand all of this via context. You're absolutely right that we know next to nothing about the subject with the single given sentence. It depends on more information.
For the Duo drills, you can usually choose "I" or "she," but that's purely arbitrary.