why does sont and elles have to have a - inbetween?
This is just a convention in French when you reverse the order of subject and verb to form a question. "Tu as un livre" - you have a book. "As-tu un livre?" - do you have a book?
Similar to how in English we say: "He would like to go for a walk." and "Would he like to go for a walk?", or "You are cold." and "Are you cold?"
How can you tell whether this sentence refers to singular or plural noun?
Kee Sont Elle
I used "Who are the girls" rather than "Who are they" to indicate gender, and it was incorrect.