I find it really interesting to see how different European languages have dealt with the whole singular-plural/formal-informal thing with second-person pronouns. In English the singular became so informal that it became pejorative, so it disappeared. In French they simply use the plural as a formal pronoun. In Spanish they have separate singular and plural forms for formal and informal, though I've heard that different dialects of Spanish do things differently. In German they used to use the second-person plural as a formal pronoun, but then it gave way to using the third-person plural as a formal pronoun, so formal "you", whether it's to one person or more than one, is actually "they".
Yeah, Spanish does it a lot of different ways.
Tú and
usted are pretty universal for informal and formal, with
ustedes as the plural in most cases. Then in some dialects there's a informal plural -
vosotros. Then in some dialects there's a singular that's even less formal than
tú:
vos. So in my husband's family, for example,
tú is sort of kind of formal, like you'd use it for your parents but sometimes also for strangers,
usted is so formal that the only time they really use it is at church where that's kind of become a pan-Hispanic Mormon way to refer to fellow church members, and then
vos is how you would address your kids or someone you were angry with.