which is the correct why of asking will you go outside?
The first : iras-tu dehors
is it because iras-tu is future tense for aller?
But Vas-tu aller dehors ? would be possible.
it all depends on the question :"are you going to go outside?" or "will you go outside?" ... don't you think ?
Duo would phrase it as, «Vous êtes en train de préparer aller dehors ?
Yes. I meant it was possible as a sentence structure, not that it meant the same.☺
It's said, but it's more spoken langage, not entirely correct, I assume. We say rather "iras-tu dehors"