in this case the subject is tacit if we wanted it to be explict, the sentence can be re-written as: Por qué (él) está tan triste? -> Why is he so sad? Por qué (ella) está tan triste? -> Why is she so sad? in this first 2 cases the sentence can be used when asking to someone about a third person who looks sad Por qué (usted) está tan triste? -> Why are you so sad? in this last case the sentence can be used when you ask some about his/her sad looking Hope It helps... Saludos

Exactly. you are right

In Spanish we rarely write the subject. We use to write/say it when we have a contrast. For example, we don't translate "he is sad, but she is happy" as "está triste, pero está feliz". We need to identify who is sad and who is happy. But normally, by context, we know who we are talking about. Imagine this conversation: -Ana no ha venido. -Está triste. -¿Y Pepe? -Está enfermo. We know who we talk about, so we don't say "he" or "she".