Ihr and du
How do we know when to use which?
To restate what Ken335502 said: "Ihr" approximately translates to "y'all" or "you guys". "Du" simply refers to a single person. "Du hast 24 Lingots." As in, Mariamarti has 24 lingots.
I hope, if you couldn't/didn't understand or wanted clarification on what Ken said, that I was helpful.
Also: In german, you have two ways to adress one person: "Sie" and "Du". You say "du" to your friends and good acquaintances. You say "Sie" to your boss and other not well known people.
If you want to say: "Your pencil" and you mean at example your boss, you say "Ihr Stift" in german. If you mean someone you know better, you say "Dein Stift". On the other hand, like the commentators before me said, "Ihr" can mean the plural.
Ich = I +++ Du/Sie = You +++ Er/Sie/Es = He/She/It +++ Ihr = You +++ Sie = They +++ Wir = We