I think "then there was a strong wind outside" should be accepted.
'outside there was a strong wind then', is marked wrong. I don't find any reason it should not be accepted.
Right, «на у́лице» means 'outside'.
But if it's literally "on the street", then why isn't on the street accepted? How would you say "there was a strong wind on the street then".
First of all, if we mean it literally, we would say "in" the street in this case. I can't explain when we say "on" and when "in," but they are not necessarily interchangeable. Second, I think the point of so many of these sentences is to teach that на улице is an idiom that means outside or outdoors.
When you use был, стал sometimes afterwards you use nominative case, sometimes instrumental, how do I know which one to use?
Also, it wouldn't accept "It was very windy outside then." It offered the translation "It was a strong wind outside then."
Well, "it was a strong wind outside then" is obviously nonsense, so that's weird. However, I would say that "windy" wasn't accepted because it is an adjective, so the grammar changes completely. The meaning is the same, but I think it is fair to reject sentences that have a different grammatical structure like this.