Translation:How the lion escaped, it's a mystery!
It does sound a bit Yoda, but it's clearly supposed to suggest the French. The idea is to teach you that "Comment le lion s'est échappé, c'est un mystère !" is a valid French sentence. If the English was structured differently, you'd structure the French differently as well, Duo would probably accept it, and you would never learn to write French sentences with this form.
On the French-to-English exercise, "How the lion escaped is a mystery!" was accepted for me 2021-02-25.
The 'APPARENT ' grammar error is that the 'APPARENT' question starts with the interrogative adverb Comment. When you start a question with an interrogative adverb then you must follow it with inversion...
But I do not see this as a question. And notice that DUO's punctuation mark is an EXCLAMATION. This is like saying "I do not know how the lion escaped". We actually use this type of 'expression of incredulity' in english all the time.
So do I..April 9/2021.
Edit: I am rescinding this agreement that it should be "how did the lion escape". I cannot, at time of writing provide any "expert premise", for the following reasoning: My self-correction is based on the exclamation mark at the end and after several reviews to try and answer questions re the NON 'french inversion' . I think that the exclamation POINT of INCREDULITY... dismisses any invitation to elaborate which is somewhat hinted at by the version "how DID the lion escape?'
It is the pithy of Hemingway's "She’s just having a bad time!" (Farewell to arms) vs
"He would have used his arms and his hands to push himself up; but instead of them he only had all those little legs continuously moving in different directions, and which he was moreover unable to control." Franz Kafka, The Metamorphosis
What bad days...It's a shame!
Duo is translating the French the way it's written and spoken. I have no problem with that. We are learning the proper and common French construction, and it doesn't always translate smoothly. But I'm here to learn idiomatic French, since I already know how to compose good English sentences.
This double expression of the subject is very common in French. It's rare in English, so literal translation helps us to learn. I'm sorry that so much of these discussions is about English while the French is mostly overlooked.
It's not a clause, no, it's a portion of the clause that has been left-dislocated. Sentences of this type aren't common in modern written English, but they have a long history.
[edit: Had a brain fart there. It's not left-dislocation, it's a fronted adverbial. But it's still grammatically correct.]
We still use them in spoken English, and yes, sometimes when we transcribe them we use a question mark. But historically a comma has always been sufficient for this, and to my mind looks more elegant. The question mark looks a little emphatic.
In any case, the comma is the only possible way of retaining the French sentence structure, which is clearly the entire point. Otherwise you would translate it as "Comment le lion s'est échappé ? C'est un mystère !" - which is a valid French phrasing, but not the type of sentence structure Duo is trying to teach. Left-dislocation is still very common in French, so you ought to learn it.
Ah, you're right!! Sorry, there are so many examples here involving left-dislocated sentence portions that i went into auto-pilot, but you're right, it's not one.
It's one of those infamous fronted adverbials.
And it's being used to make the sentence topic-prominent, which is also common in French.
Not Sitesurf.. but here is what I have found.. "The lion, did he escape" == "Le lion, s'est-il echappé" But "How did the lion escape, it's a mystery?"= Comment le lion s'est échappé(INTONATION), c'est un mystère" OR " Comment s'est échappé le lion (interrogative adverb (EXCEPT POURQUOI)+INVERSION ...OR "Comment, est-ce que , le lion s'est échappé...INTERROGATIVE ADVERB + est-ce-que + plus normal word order". _
Basically prefix the question with the adverb nullifies the use of your construct.
Here is a GREAT resource by the respected Le Robert which provides a TON of sub links which helps in understanding 'indirect' questions. It is AMAZING but needs a scholar's mindset not the knee-jerk "DUO is wrong"/"I am a native....progeny" obduracy. Look at the sublink for interrogative adverbs....so many good examples.