As a linguist, and having grown up reading the King James Bible and Shakespeare, I get extremely irritated when ignorant people goof around with “thou” conjugation and add “-eth” or “-est” to adjectives, nouns, wherever they think it might be funny. There is a mystique associated with “thou” because of its use in the King James. But its use was not complicated, although its conjugation can be. “Thou” was originally the singular form, and “you” plural. With time, “thou” became the familiar form and “you” the respectful form. By the late 1600s, “thou” fell into disuse, and now we use “you” for everyone.
Spanish has a more complicated pronoun history, and remains more complex than English. In school you were taught “tú” and “usted” for “you”. “Usted” conjugates with “él/ella” and is the respectful form, “tú” is the familiar, paralleling “you” and “thou”.
However, in real life, vast sections of Latin America use a third form, “vos”. If you hear it in a song, the singer or songwriter is probably Argentinean. But in conversation, you’ll also hear it in parts of Central America, Colombia, Venezuela, Ecuador, Bolivia, Chile, Paraguay, Uruguay… It’s rare in the Caribbean, and in Spain “vos” is only used in court.
I learned Spanish in the southwest of Colombia (Pasto and Puerto Asís). Nearly all of our conversation used “usted” (or “busté” as my buddy Pedrito would say). When we moved to Medellín, we had to start using “tú”, plus they also used “vos”, which we had never heard at all before. This took a lot of getting used to.
Roughly speaking, “vos” is familiar/casual (like “dude”), “tú” is familiar/standard (like “you”), and “usted” is respectful (like “sir”). People are not necessarily consistent, plus it varies by region and even family. My wife Alicia, however, is extremely systematic and consistent in how she uses the terms of address.
She always uses “tú” with her son. He also addresses her with “tú”.
With shop girls and street vendors, she uses “vos”. They will respond with ”usted” or “tú”, depending on where they’re from.
With her siblings, she uses “usted” if it’s a serious conversation (like if she’s lecturing or advising them, which is frequently the case). She uses “vos” for casual or joking conversation with them. (Her siblings use “tú” or “vos” all the time, and rarely use “usted” with family.)
With friends, she uses “tú”, and occasionally will joke with “vos”.
With me, she always uses “tú”. I sometimes use “vos” with her, especially if I’m being silly. Once I used the “usted” imperative form (asking her to hand me something), and she was hurt. (I grew up using the respectful imperative and didn’t learn the familiar “tú” imperative until college. I still have to think about it sometimes.)
Here are examples of the three in imperative (command) form. The respectful imperative uses a subjunctive conjugation (another feature that has largely disappeared from English). The familiar imperative uses the same form as third person singular present tense, excepting a few irregular verbs. The “vos” form is usually a modification of the “vosotros” form (plural of “tú”, used primarily in Spain), with some exceptions, as you can see below. Sometimes it’s appropriate to add a pronoun suffix (-te, -se, -os, -me):
Usted Tú Vos Vosotros
¡Venga! ¡Ven! ¡Vení! ¡Venid! “Come!”
Coma Come Comé Comed “Eat”
Vaya Vete Vete Id ”Go”
¡Cállese! ¡Cállate! ¡Callate! ¡Callaos! “Shut up!”