I put when does it start raining, which is better than their flipping solution.
"when does it start to rain?" is also accepted - and perhaps a more complete question when presented by itself
The solution "When it starts raining" is a sentence fragment, it doesn't make sense.
Without context it doesn't make sense, but it could work with an appropriate context—something that seems common on duolingo.
"When do we give up walking and take the bus?" "When it starts raining?
I'm not sure whether the Portuguese needs such a contrived context as this translation, however. Given that it accepts much more complete sentences for this question, I'm inclined to think not.
This is wholly incorrect English. The meaning is clear in Portuguese, and it should be translated as "When does it start to rain?", or the equivalent.
I said “When does start the rain?”: I imagined a situation when you're waiting for a particular rain, like the rain that will appear in a given time. Can't you use a definite article in such case?