I have never heard this English version as a saying. It's another case of "try and guess what duo wants you to write".
It is a phrase used in English but the hints for 'duro' do not suggest 'firm' Another English idiom for this would be 'I held my ground'
Asking people to guess at idiomatic phrases without prior introduction is ridiculous.
It is discouraging to participate in activities when you know you cannot succeed, and frustrating to have to repeat entire lessons over and over because of unanswerable questions. Perhaps it is intentionally structured that way, as you say, maybe to force repetition or to make definitions stand out in your memory by being associated with anger.
Sometimes I yell at the computer, but more often I take "a moment of zen" as my Italian instructor used to say. And I'll bet you know a lot more now than when you began Duolingo. Just remember that - and smile!
We learn by doing, and failing, then doing again. Otherwise we wouldn't need Duolingo for training - i would just read and study. But who wants to do that anyways, right? :)
It's not ridiculous - it's how you learn. You get it wrong, and then the next time you get it right. If you get it wrong a few times before you learn it, all hope isn't lost
Just out of curiosity, would "I held tough" be accepted? It's the same basic saying, but tough is closer to the meaning of duro.
I'm just frustrated that I lost hears by saying strong as a mule a few exercises ago and now I lost a heart for I have remained stubborn