"Die Maus spielt mit einem Bären"
"Die Maus spielt mit einem Bären" == "The mouse plays with a bear"?
"Bären" is the plural form of "Bär" (bear) and "einem" (one) indicates singularity of the object. So this phrase is saying "one bears", and it is conflicting (confusing), at least in English.
Is this a special rule (in a lesson that I have missed) in German? E.g. it must be in plural form after "mit"?
I am not a specialist, but I think it is no mistake.There is actually a category of masculine nouns whose endings change according to the case. Bär is one of them. They are called masculine weak nouns. Maybe this link will be useful: http://www.vistawide.com/german/grammar/german_nouns03.htm
You're right domidomi. I goofed! Another one to look out for in these lessons is the word Kinder, which takes an -n in the dative case as well.
It looks to me like a mistake by Duolingo. Those are all over the place by the way!
MonsterBoy, all german nouns take an "n" in the dative plural, unless they end in a letter like "n" or "s", which don't take the "n" for pronunciation reasons. "Den Kindern, den Leuten, den Brüdern",but "die Frauen, den Frauen"
Thanks very much. It is very enlightening, but unfortunately it also means another tricky rule to look out for (argghh). I guess a good German dictionary will be of great help here...