This is in the context of telling time, where you say "È l'una" for "It is one" and "Sono le due/tre/../dodici" for "It is three/.../twelve", as "una" is singular and all other numbers are plural.
Just to explain further based on what thenino85 had explained. I believe it's easier to understand when we are looking at the whole sentence: Addeso le ore sono le undici. Literally Now the hours are eleven, and after some paraphrase, it becomes Now it is eleven (o'clock).
Oh, now I understanded. I didn't know the whole sentence. Ya, now it's easier. Thank you!
You can look at the comment of thenino85 above, or the one I made below. Hope it helps.
Don't forget 'sono' can mean 'I am' or 'they are.' [Io] sono = I am, [Loro] sono = they are. As the next word [le] is plural, it tells you that this 'sono' must be plural. Literally, 'they are the eleven' or in English 'It is eleven o' clock,' - and just as logical!!!
eg. Sono un uomo, sono uomini - I am a man, they are men.
I hope that helps. 17/01/2015
So this sentence could be either now it is eleven o'clock (which is accepted) and now they are eleven? which is not accepted. I understand that they meant us to tell the time but is the other a fair translation and if so is there no way to know which one it is except within context?
Yes. Context is definitely key in this exercise. The only singular hour [one o' clock] translates in the singular - e` l'una - and all the others are plural - sono le due - etc., etc.. The thing is that although 'they are eleven' is a valid translation, it is not how to say the time in Italian. You just can't translate the time word for word, either way. In English we say 'it is one of the clock,'[contracted to o' clock] which I'm sure is just as confusing to Italians who are learning English. I hope that helps, ciao. 17 June 2016
Because the way the sentence is written (or spoken) tells us that we are talking about the time, and Duolingo feel clever if they can trick you. Now they are eleven = Adesso loro sono undici.
I can't understand, it's so confused. And how could I know that it was, if nothing was explained about?
DL is once again making me waste my time by not accepting: Now it is eleven in the morning, or Now it is eleven o'clock. Please.