In modern English, second-person pronouns have the same form in the singular and the plural — “you,” for instance, can mean either one person or a group. (I set aside the now almost entirely archaic “thou,” and the regional “y’all.”) The same is true in some other languages, at least as to the formal second-person singular. (The informal second-person singular, equivalent to the archaic English “thou,” survives in at least some of those languages.)
But what second-person pronoun form is actually different in the singular and in the plural? It might be obvious to most of you, but I just thought about it a few days ago, when talking to my 5-year-old, and realized that I’d never consciously noticed the difference before (though I’m pretty sure I always use both forms correctly).