"How beautiful you are!"
Translation:¡Qué hermosa eres tú!
Rejected "Qué hermosa tú eres". See "https://twitter.com/entreleitoras/status/686361431311257600". Reported 08 April 2018.
Ah, there's this comment--I've been wanting to correct it for awhile but couldn't find it again. I'm happy it caused you to think, Ronaldo, but hope you thought in the correct direction. The rule I noted is correct, but its application here is NOT! Qué in Duo's sentence is an exclamatory adjective, not a "question word." Perhaps the best guide is the one SGuthrie provided: https://www.lawlessspanish.com/grammar/adjectives/exclamative-adjective/
"Que tu eres hermosa" is like saying, "that you are beautiful," so you could use it in a sentence like, "Les dije a mis amigos que tu eres hermosa." If you change the word order, just like in English, you get, "How beautiful you are," or, "Que hermosa eres tu," or, "Que hermosa eres." I hope this helps.
"Tan" means "so," so saying "Que tan hermosa eres," is like saying, "How so beautiful you are." Therefore, if you omit the "tan" and write, "Que hermosa eres," or "Que hermosa eres tu," you've nailed it. If you were translating, "You are so beautiful," you would write, "Tu eres tan hermosa," or, "Eres tan hermosa."
If you were translating, "You are so beautiful," you would write, "Tu eres tan hermosa," or, "Eres tan hermosa."
That's a literal and wrong translation, "You're so beautiful" can translate to "Eres muy hermosa" or "Qué hermosa eres". In Spanish tan is not used as a submodifier, even though Duolingo likes to teach that.