"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.
Por favor, equipo de Duolingo; pronuncien bien las vocales; en español, cada vocal tiene un solo sonido (y solo uno) y ninguna de las cinco vocales entre sí se parece a las otras. O tomen un curso de dicción. Si en español, le cambian el sonido a las vocales, sabiendo que tienen un solo sonido; que será en otros idiomas, donde las vocales tienen varios sonidos.
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
Please enunciate the endings. It sounds different on fast and slow.
This is so frustrating when you are listening carefully and can't understand. whether it is an "a" or an "o." I can't tell you how many times I have been caught on this. If you hear both A and O on fast and slow and you pick one. Which one do you pick?
THIS SPEAKER IS VERY VERY VERY UNCLEAR IN SPEAKING! !! I KNOW VERY WELL HERMANA IS FEMININE AND HERMANO IS MASCULINE [SISTER AND BROTHER!] THIS SPEAKER DOES NOT - ABSOLUTELY DOES NOT NOT SOUND LIKE SHES SAYING THIS PROPERLY!!! PLEASE REPLACE THIS SOUND BITE FOR AND ACCURATE PRONUNCIATION! PLEASE!