There's something wrong with the numbers in German. We have learned "eins", "zwei", "drei", etc... But when it comes to using the number one, then "eins" is virtually never used, the indefinite article "ein/ein/eine" is used instead.
Any Germans here? How would be "Name two!" or "Name three!" in German? Are there any situations where you could actually use the number one (eins) instead of the indefinite article?
You could most certainly use "eins" when you are counting something, any random object. But you can use the articles that correspond to the gender of the object you are counting. Also, in commands like these, you would usually know what "one" thing you have to mention, so you would use the article here. If that makes sense.
Well in this case we know what we are talking about. We talk about mice so the proper answer would be "Nenne mir eine", because mice (ger: Die Mäuse is female). But the meaning in this case is more like you don't trust the statement of your friend and want him to give you an example.
In this case especilly with the "Sie" it sounds more like a formal question and someone wants an answer.
For example (In an interview for a new job):
We have talked about your weaknesses.. Name one.
Wir haben über ihre Schwächen gesprochen. Nennen Sie eine.
I realy hope this somewhat helped you.
What a lot of you should think about is if this were to be a complete sentence, it would be (taking Flenzil's e.g.): Name (me) one mouse and since mouse = die Maus (feminine), it would be Nenne mir eine.
Similarly, as wchris2 says with his last example: name one weakness ...that would be = Nennen Sie eine (Schwäche); thus, (for short): Nennen Sie eine because it's die Schwäche (singular - nothing to do with plural). If unknown, like Duolingo's example, it's masculine.