"She and her brother are playing."
Translation:Zij en haar broer spelen.
Zij is stressed in this sentence. If you say the English sentence out loud a couple of times, you'll notice that you're putting a little bit of stress on She as well.
I respect your authority on the determination, but I still don't understand the reasoning, linguistically. Why can some sentences be ze/zij, but not this one?
El2theK already answered your questions thoroughly and provided resources in the form of website links to help you understand the difference. There's not much more I can add to that, other than to say that Ze simply isn't correct in the above sentence. If you are really interested in learning to speak Dutch, you will eventually understand the stressed and unstressed forms of certain personal pronouns. It just takes time and practice.
Also, as I mentioned to JonKScott, even though English doesn't have stressed and unstressed forms of personal pronouns, "If you say the English sentence out loud a couple of times, you'll notice that you're putting a little bit of stress on She as well."
No, you are pointing someone out emphasising hence you can only use zij here.
If you respond to the question: "What are they doing?" Then <<playing>> gets stressed. Or, if you respond to the question, "Is that girl playing with her sister?" then <<brother>> gets stressed. The lesson on stress points out that it's all context-dependent. And because you can't infer context from a single, decontextualized sentence, ze/zij should always be acceptable
Again, you are pointing someone out, it does not matter if you put stress on another word in the sentence. Because you are pointing someone out you put emphasis on she in this case and hence you can only use zij.
So yes, you can infer that from a single sentence in Dutch without any other context. Or to say it differently, using ze in this sentence is completely awkward and wrong in Dutch..
If that were so, you could only start sentences with zij, as you would always be pointing someone out, theoretically. Are you a native Dutch speaker? How do you know it's awkward and wrong in Dutch?
Yes I am a native Dutch speaker, born and raised (apart from a few years abroad). How do I know? Well that I'm a native Dutch speaker does help. Dutchesse is also a native Dutch speaker by the way.
However, regardless of that it seems you don't quite yet understand the difference in use between ze and zij.
Perhaps these examples will help:
Or have a read through this, and the comments: https://www.duolingo.com/comment/3734337/Grammar-Jij-vs-Je-Zij-vs-Ze-Wij-vs-We
OR, if you respond to the question, "Are she and her brother speed-walking?" then the emphasis is on <<playing>>.