"Tu" is when you're referring to a close relative, or a friend; it's a personal word.
"Su" is used when speaking with an aquaintance, or peer; it's much more formal.
The word "usted" is also a formal word and means the same thing as "su", however you'd use this when talking to someone older than you.
Lol. "Tu your su" sounds like "tie your shoe" of you're brushing your teeth and trying to talk.
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"Tu" is "your" unless there's an accent over the u. In that case, it is "you."
When you answer a question incorrectly, you will find a "Report" button in the "Correct solutions" ribbon at the bottom of the screen. You can report this issue there each time that it happens.
If this is happening for every question and you are answering the question correctly, hop on over to the troubleshooting forum. https://forum.duolingo.com/topic/647
i think the sound of hermana and hermano is the same when spoken here. It all sounds like hermana to me- whether slow or fast