German has three words that represent the English word "you."
du = you singular, informal ihr = you plural, informal
Sie = you singular or plural, formal
You would use informal forms when speaking with friends, teammates, family members or children. You would use the formal form when speaking with people that you have just met, people you use Mr. or Mrs. with, work colleagues with whom you aren't on informal terms with (and this could be people you've worked with for some time).
Sie (always capitalized when used as the formal "you") is conjugated the same way that third person plural is conjugated. In other words, it uses the same verb form as sie = they.
So you have three German words for "you" and there are also three possible meanings for sie/Sie. sie = she sie = they Sie = you
You can tell the difference between she and they by the verb form. Sie sieht = She sees. Sie sehen could be either they see or You (formal) see.
I suppose it would require a very specific context. "But I thought they were blind!" "Well, you were wrong. They see." Or maybe: "I explained it to them, they said OK. They see." Or maybe even: "What do eyes do? They see." Seeing and looking are about as different as hearing and listening, after all.
That is a good question! Y'all (you + all) can be singular or plural, so it could be translated as du, ihr or Sie. “All y'all” can be used as “ihr” or “Sie”. As a native English speaker (US) I would advise English students to use “you” instead of y'all. It is used mainly in the southern US. It is good to know what it means and recognize it, but I don't know if "y'all" is used in the UK and other English speaking countries. If you travel in the American south, you will hear "y'all" in everyday conversation. It is seldom used in the rest of the US. I think that most Americans will understand you if you say “y'all”, but all of us will understand if you say “you”.
To hear y'all used go to: https://www.youtube.com/watch?v=vihYkEAQ_DY If the URL does not work, Google: Friday Night Lights: All the Y'alls from Tami Taylor