"¿Tú no te sientas?"
Translation:Aren't you sitting?
"Why don't you take a seat?" is: "¿Por qué no te sientas?. "¿Tú no te sientas?" coul be used, for example, when you see someone standing up and then another person says to all of you to sit down and the other person do not sit down so then you ask to the other person "¿Tú no te sientas?", like if you were asking "aren't you going to sit down?" or, like it says above this, "aren't you sitting?". I hope you understood this. (Please correct me if I made a mistake).