In French, it is similar in the way that when you want to say, "I want some (of it)" you have to say, "j'en veux" - meaning I want some (with "en" replacing the "of it". So instead of being able to say, "Are we sure?", you need to keep what English lost which is the thing of which you are sure. So you have to say, "Are we sure about this?"
"En" is a pronoun with a basic meaning "of this/that" or "of these/those". It is used extensively in French. It always goes with a verb as here. Literally "Of this are you sure?"
It's not to be confused with "en" as used in "en France" (in France) or "en marchant" (while walking) which is a different word.
I think Duolingo should have given a better explanation of the word here.
Actually, JeanPaul_G's version is the correct one. The declarative phrase would be "Nous en sommes sûrs". (source: native :) )
"Nous sommes en + something" isn't actually incorrect per se, but in this case en would be a preposition and the phrase would mean "We are in + something".
Nous sommes en France = We are in France,
Nous sommes en cours = We are in class.
No, that is not correct. It has been a long time since I have taken a class in grammar and I have forgotten about placement of objects, prepositions, etc., in relation with verbs. All I can say is that the sentence in the exercise "En somme-nous surs" is correct. Another way of translating the sentence could be "Are we sure about it." The French would still be the same.
"Somme-nous surs?" is an incomplete sentence in French. You have to say, " en somme-nous surs?" to be understood. In English we can get away with, "are we sure?" and is considered a full sentence, even though it is not in a literal sense, as 'of it' or 'of this/that' is implied already.
It's weird because "En sommes-nous sûrs?" could also mean in English "are we safe?" Yes I know it's totally different but some sentences in the french language could mean more than one meaning so you should let the learner understand that (one sentence = more than one meaning). and thanks for your good job at duolingo.
« En sommes-nous sûrs ? » => "Are we sure about it/this ?"
« Sommes-nous sûrs ? » => "Are we sure ?" (or "Are we safe ?" - in the sense of being reliable, you could see that in a mafia movie, one character speaking about his own team to his boss : « Sommes-nous sûrs ? Bien sûr que nous le sommes ! » "Are we safe ? Of course we are !")
In fact, you will hear many French people saying it that way, instead of Duo's correct version, especially in casual talk, but I would refrain from using it in a more formal environment. The only thing is that it becomes ambiguous with the affirmation: « Nous en sommes sûrs! » ("we are sure of it"). So you have to realize that the tone is crucial here.