"Everyone has to die."
Translation:Todo el mundo tiene que morir.
It depends on how you translate "everyone". If you use the phrase "todo el mundo", then you need "tiene" because that phrase is grammatically singular (hint: there's no 's' on the end of todo); just like we say "everyone is here" instead of "everyone are here" because "everyone" is grammatically singular even though it refers to more than one person. But if you translate "everyone" as the grammatically plural word "todos", then yes it would be tienen ("Todos tienen que morir").
Although todo el mundo refers to many people, it's gramatically singular, which is why it uses tiene (if it was grammatically plural there would probably be an 's' on the end).
It's similar to how we say "everyone has" (singular form: "he/she/it has") and not "everyone have" (which would be the plural form: "they have").
Literally, yes. But todo el mundo is the Spanish way of saying "everyone"/"everybody", so that's the most natural translation.
Just like how "me llamo..." literally means "I call myself...", but we translate it as "my name is..."; or "me gusta [x]" means "[x] pleases me" but we translate it as "I like [x]".
Did you leave out the "que"? Sometimes when you miss part of the preferred answer, they give you a completely different answer instead of correcting you to what they are using as the preferred answer on the discussion page. You can always look here to see how your answer was wrong.