"Then why are they here?"
Translation:Allora perché sono qui?
The reason for that is actually inherent to the language itself: in English "then" is used to express both a temporal sequence, e.g. "first study then play" and a logical sequence, e.g. "if he's late, then he'll miss it", and also a general past, e.g. "back then". In Italian "allora" only covers the last two, while for the first one you have to use "poi". In this particular sentence some sort of "if" is implied.
What Duo wanted was allora perché loro sono qui/qua. I don't know why you can't put loro after the verb. I get the impression that there is a change is linguistic attitude about placement of the subject, and both Italian and Spanish are moving rapidly towards relying on vocal inflection rather than subject-verb orientation to express a question or a declaratory sentence.
Some of the comments have indicated this change. Sometimes Duo lets you reverse subject-verb for a quesiton, sometimes not. It's all really murky.
I can't speak for Spanish, but Italian has always relied on vocal inflection to form questions: word order is totally unrelated to that, and only used to express emphasis. In this sentence, "sono loro" doesn't express any kinf of emphasis and is simply weird: if the question was "are they the guests?" then "sono loro gli ospiti?" would be a valid word order, but that's true with or without the question mark (i.e. "sono loro gli ospiti" is a "they are the guests" with an emphasis on they).
Qui/qua both mean here, while lì/là mean there; and for the record, there's also costì and costà, but you'll likely never hear them outside of Tuscany.
The meaning of each is as follows:
- Qui: right here where I am
- Qua: in my general proximity
- Costì: right there where you are
- Costà: in your general proximity
- Lì: exactly over there
- Là: over there
In actual speech, most Italians don't mind the difference between qui and qua and between lì and là that much, usage depends more on personal preference; when in doubt, use qua for here and là for there, they're the most common, especially in idioms.