"Why are the girls tired?"
Translation:¿Por qué están cansadas las niñas?
The issue is not whether their state of being tired is a fact, but whether it is a permanent, life-long condition or a temporary condition. Soy cansado = I am tired all of the time, all my life estoy cansado = I am tired right now In either of my situations, my current condition (tiredness) is a fact
Estoy and soy are both the ''I am'' forms of the verbs, which both mean ''to be.'' However, they're used in different circumstances. Soy refers to something that you are forever, that is an inherent state. I am a girl, I am a parent, etc. Estoy is used for location, feeling, or temporary states. So ''Estoy en la biblioteca" means ''I am at the library''--you're not going to be at the library forever, just for a little while. ''Estoy triste,"" means you're sad, but you're not sad forever.
This type of question is typically formed by reversing the order of the subject and the verb. The adjective immediately follows the 'to be' verb (so long as neither is modified), and thus comes in between. Time words like 'hoy' still come at the end. Las niñas están cansadas hoy. The girls are tired today. ¿Por qué están cansadas las niñas hoy? Think of it as: Why are they tired, the girls today? I should also point out that 'las niñas cansadas' means 'the tired girls.'