English is a difficult language in many respects, but one way in which it is blissfully simple is that it has only one word for "you." Singular, plural, formal, informal, it's all the same. There is, of course, the archaic "thou" and the regional "y'all," but a non-native speaker will always be perfectly safe just using "you." Not so in other languages. German has its du (singular informal), ihr (plural informal), and Sie (formal, regardless of number). Spanish is even worse because it has regional variations. Usted is the singular formal around the world, but the singular informal is tú in some countries and vos in others; ustedes works for the plural formal and informal everywhere except Spain, where vosotros is used for the latter. As for Mandarin, in over twenty years I'd never heard my parents use anything but nǐ, so I was shocked to learn halfway through my Pimsleur Mandarin II course that there is also a respectful form nín.
The grammar associated with all these different forms can be a little daunting at first, but it's surmountable. Far more difficult to master, in my experience, is the cultural aspect of when to use the formal and when to use the informal. Some cases are obvious, of course---your best friend is tú, your doctor is Usted---but there are many others which are uncertain. One wants to be wary of addressing someone informally for fear of showing disrespect, but using the formal when the informal is appropriate can be ridiculous or even cause offense. What are we to do, we poor English speakers who have grown up without having to make this decision constantly?
Last week I decided to settle this once and for all by interviewing my German coworker at length about his usage of du and Sie. How does he address his coworkers? Our company's clients, in his former position on our Europe support team? His parents' friends? The first thing he said, to my dismay, was that it's something he struggles with all the time as well, and that there really are no hard and fast rules. He uses du among his colleagues, but fellow tellers at a bank would almost certainly use Sie in accordance with their more formal work environment. Interestingly, our clients addressed him as Sie, but less knowledgeable staff (regardless of seniority or age) were called du. Apparently it really is about respect, the lack of which can be made painfully obvious with a simple change of pronoun.
I figured that as long as I was doing this research, I'd also ask my parents and Chinese coworkers about the use of nín. The general conclusion was that it's really pretty rare except when addressing the elderly, which I fortunately don't have to do in Mandarin very often. And, too, the great similarity in sound of the two forms allows me to hope I'd be given the benefit of the doubt should I ever commit a faux pas. Perhaps there's hope for us linguistic egalitarians after all.