"It was one of you."
Translation:Fue alguno de ustedes.
But Espanol consists of both the Castellano accent (vosotros) and Latin American accent (ustedes). Using the flag of spain would include the specific type of spanish we are learning here. There is no flag that represents all of Latin America but the flag of Spain does represent the language spoken in LA.
it is not because of that.
the key word is 'FUE' which describes a past event while referring to a person: it was one of you=fue alguien de ustedes.
vosotros: the use of 'vosotros' is correct, but 'ERA' is incorrect in that sentence because 'Era' describes a past event in which you are included : it was one of us=era uno de vosotros/nosotros (vosotros can be used for 'uds.' and 'nosotros'. especially used in old Spanish)
Your reasoning for era is totally incorrect. Era is the third person imperfect of ser (although it is also the first person imperfect) So the difference between fue and era is like the difference between the preterite and the imperfect of any verb. I think most times people think of this sentence it is in response to a question about who did something and it normally would be preterite. But imperfect is possible. Fue tú, siempre era tú. It was you, it always was you.
"Lo" is "it" only when "it" is an object.
When "it" is the subject of a sentence, or not an object, "it" is included in the conjugation of the verb.
"It is raining" = Está lloviendo
"It is closed" = Está cerrado
"It is blue" = Es azul
I didn't do it = No lo hizo
Did he say it? = Lo dijo?
Fue = he/she/it was
Fueron = they were [ustedes fueron = you (plural formal) were]
• It was . . . Fue
• one . . . . . uno | alguno [someone/-body; anyone/-body]
. . . . . . . . . . . una | alguna [these only in a context of an 'all female' group]
• of . . . . . . . de
• you . . . . . . ustedes [plural formal /Spain; informal + formal /Latín America]
. . . . . . . . . . . vosotros [plural informal /Spain]
In the beginning of the sentence the verb has to be conjugated and translated in the singular form, as "it was" (to whatever action or "form of being" it's pointing at) one/somebody singular from a group of several people. The end of the sentence is pointing at the group, therefore "you" has to be translated in the plural form.
I have to google the question to find the discussion! Because since I started this course today it's been leading me to empty discussions like this one: https://www.duolingo.com/new-comment?fromLanguage=en&learningLanguage=de&sentenceId=7e4791f154171bc63d2a1cf2d6deefe7 I can't even report it!
I think you need uno not un because it appears by itself. See eg
As JuevesHuevos said above, "lo" is "it" only when "it" is an object. "It" is included in the conjugation any time "it" is the subject.
It is raining == Está lloviendo It is blue == Es azul
I did not do it == No lo hizo Did he say it? == ¿Lo dijo?
More in JuevesHuevos' post