There are several ways the English "You are responsible" can be expressed:
- Tu es responsable (singular, familiar)
- Vous êtes responsable (singular, polite)
- Vous êtes responsables (plural)
I.e., there is nothing inherently "wrong" with "Vous êtes responsable" unless it is that Duo does not accept the other variations. There are numerous cases of correct responses being disallowed and if that is the case here, it should be reported.
Can it be used to mean 'responsible' as that word might also be used in English, to mean generally trustworthy, mature, prudent, etc? Or is it restricted to the meaning of 'responsible' as 'accountable'?
It can mean responsible, as in "in charge of", as in "liable", also "responsible" as a character trait: Ce n'est pas très responsable de sa part = That is not very responsible of him; Elle s'est toujours comportée en personne responsable = She has always acted responsibly. http://www.larousse.fr/dictionnaires/francais-anglais/responsable/67943
I just tried to report the issue in the question where you asked to pick given "Vous êtes" whether to use "responsables" or "responsables," (I picked "responsables" and was marked wrong.) and the option of my answer should be accepted was not there. So, it is an UNREPORTABLE error.
Thanks for letting me know. The error is that the "form exercise" which asks you to select one correct answer actually has two correct answers (responsable and responsables) but considers the plural form to be incorrect. I have disabled the form exercise.
I believe that "vous" can be singular or plural. Is my understanding correct?
Does this means that it is your fault that something has happened, or that you are in charge of making sure nothing will go wrong? Or both?
I don't know. I would guess that he is saying it is your fault, but I don't have any proof.