I found out that many of the Duolingo courses just use "you" to translate both singular and plural 2nd-person pronouns in other languages to save space. I don't really agree with this practice, since it doesn't help reinforce the difference to English-speakers - and judging by some of the comments, many English speakers do get confused - but there it is. Some of the trees, such as the Swedish one, do allow "you guys" as an accepted translation of their 2nd-person plural pronoun, but this practice hasn't been expanded to all of the courses yet.
Do you like milk? = Ви любите молоко? (plural)
Do you like milk? = Ти любишь молоко? (singular informal)
Do you like milk? = Ви любите молоко? (singular formal)
German has the same construction, by the way. Of course you can say "you guys" in some informal cases but I think it is not necessary.
This is exactly my point - unless it's specifically explained to them, an English speaker is not going to understand the difference between those three Ukrainian forms, especially if they've never studied a language with T-V distinction. I've already seen some English-speakers ask questions like "wait, I thought 'do you like milk' was Ви любите молоко, why is it now giving me Ти любишь молоко?" Of course, the difference can be explained, but the point of DuoLingo is to acquire linguistic structures naturally through the course lessons, not rely on sporadic answers in the comments, or even the grammatical explanations at the beginning of the lessons, which are unavailable to mobile users.. "You guys", far from occurring in just some informal cases, is the standard 2nd person plural pronoun in spoken English across much of North America. Bare "you" to mean 2nd-person plural is actually on its way out in North American English, unless it's anaphorically referring back to a first instance of "you guys". So, the largest segment of the English-speaking world now has an established 2nd-person plural pronoun, and I think it'd be helpful to English-speaking learners of Ukrainian and the other languages on here to have their "you/you guys" distinction connected naturally to the singular/plural distinction for 2nd person pronouns in their target language.
You is the plural form and "thou" was our singular form. I respectfully disagree. "you guys" and "You all" are informal and limited to certain areas of North America. When there is a group, we will name the person when there is just one that we are talking to. "John, will you please close the door." The whole class does not have to get up to close the door that way. Even in a press conference we will describe the person if we do not know his or her name, the speaker will say "Yes, you in the blue shirt, I will take your question now." Speaking to the whole group, "you" is enough.