How do you know if you is tu or voi?
When DL tells you to write something like "You are late," in Italian, how do you know whether they want the "tu" version "sei" or the "voi" version "siete" of the sentence written in Italian?
I gave an example... "You are late." That's the example, and that's all they give you to translate into Italian. If DL gave me more context, I would certainly be able to know which one to use, because it would be clear in the context, but when they say write in Italian: "You are late." Or "Are you going?" and that's all they give you to go on, it's impossible to know whether it should be Tu or Voi.
For "You are late" Duolingo should accept no less than four different translations:
(Tu) sei in ritardo. (single listener, plain)
(Lei) è in ritardo. (single listener, formal)
(Voi) siete in ritardo. (multiple listeners, plain / formal)
(Loro) sono in ritardo. (multiple listeners, very formal).
Since the exercise does not provide a context of speech, you cannot tell whether 'you' is singular or plural, nor whom you are speaking to, so all four versions are fully correct.
I found guessing on Duolingo much easier than choosing in real life. I was introduced to an Italian friend of friends in their home. The next day, in the same home, I went for 'come stai?' Oh dear, monumental mistake followed by a lesson on using Lei. I didn't open my mouth again!
Dear Helen, don't worry about that. Do as I do. I speak all that comes to my mind and let them sort out the grammar :-D. As long as they understand you, it is all that matters. They know from your body language and smile that you don't mean to offend them even if you miss dare di Lei. I found that people are very forgiving when they see your effort speaking their language. You can't learn anything without mistakes.
Tu = You (Singular) Voi = You all (Plural)
EX: Voi siete felice. = You all are happy. Sei felice. = You are happy.
However, in English, there's no real correct subject for the 2nd person plural, at least one that is commonly accepted. So "you" in English can mean 2nd person singular or 2nd person plural depending on context.
To answer the question, "You are late" can be both translate to "Sei in ritardo", or "Siete in ritardo".