"du" and "ihr" vs. "Sie"

du and ihr vs. Sie

The proper German pronoun for "you" depends upon your closeness to the person you are addressing:

Sie and its related forms are used when addressing one or several persons whom you would normally call by the last name.
Du and its related forms are used when addressing one person (ihr more than one person) whom you normally would call by the first name.

Herr Schmidt, kennen Sie meine Frau?
Robert, hast du meinen Hut?
Kinder, ihr seid zu laut! Mr. Schmidt, are you acquainted with my wife?
Robert, do you have my hat?
Children, you are too loud!