Why is 'au' used for 'with a' ball and not 'with the' ball? Would have thought 'avec une' was better?
Sorry, Duo, that is an Americanism! In English we play with a ball.
Yes. British children often "play with a ball." But equally often, they "play ball" together..
. .so, while there are a lot of exclusively American word forms throughout Duo's translations, this really isn't one of them.
Update: Duo has now rephrased, to include "with a." Hmm.. They certainly keep us on our toes, don't they?