"Are you outside?"
Translation:Bạn có đang ở ngoài không?
According to the advanced learner I have access to, the "có" is always required in yes/no questions to be grammatically correct. However, it's perfectly understandable and often more natural in relaxed speech to omit it.
He said to compare it to the English do/did. It's correct to say "Did you eat yet?" But often people simply ask "You eat yet?"
I talk with natives on a daily basis (though not the northern dialect duo teaches) though im not actually native and though you are technically supposed to use có with không it should not be marked wrong as it is rarely used in conversation and duo doesnt use it itself half the time. And there are also a lot of times where không can be used without có or even shouldnt be used though im not sure what they are exactly yet