Du and Ihr?
When do you use "Du" and when do you use "Ihr"?
angiedaytripper's answer is partially incorrect:
-- "du" is the informal, singular 2nd person, as in the English "you"
-- "ihr" is the informal, plural 2nd person, as in the Texan "y'all"
-- "Sie" (capital 'S') is the formal singular 2nd person, used in formal contexts, to strangers, co-workers, professors, etc.
-- "sie" is the plural 3rd person, as in the English "they" (as compared to singular 3rd person, "er"/"sie"/"es") -- yes, it can be ambiguous.
'Y'all' is a perfect comparison. Use 'du' for family, friends and people that are younger than you. Use 'sie' for strangers and co-workers and teachers etc. (formal/informal)
Ihr - is the collective 'du (informal you)' and 'Sie' (with a capital 'S') is the collective of 'sie (formal you)'
We used to have these distinctions in English, think 'thee and thou' in Shakespeare, but they're only found in regional dialects now.
Here's a site showing tables for the different personal pronouns and when to use them. They're easy to get confused so it's best to try and learn the rules!
Deklination von Pronomen https://deutsch.lingolia.com/de/grammatik/pronomen/deklination