Personal pronouns in Spanish are different to the English language system. But, what are personal pronouns? They are a category of words that we use to refer to a noun that is not explicit in the sentence. As it happens in English, if we were speaking about John before, there is no need to say "John" all the time; we can use he or him instead.
Well, in Spanish, we have a pronoun of courtesy (usted and ustedes) and a second plural pronoun (vosotros). In addition to this, our third person plural forms are masculine (ellos) and feminine (ellas). These are the main differences between both languages. However, the biggest problems with personal pronouns in Spanish are the dialectal differences. When should we use tú, vos or usted? Let’s see.
The history of these personal pronouns in Spanish
The form tú is the original pronoun that the Spanish language inherited from Latin. It was used in all situations until the 3rd century, when a new form appeared: vos (from the second plural person). Speakers used vos only for formal situations and to address to dignataries: the king, the nobles and the clergy.
This situation was stable until the 16th century. However, the constant use of vos started to lose its meaning of courtesy because it was not so exclusive as it was at the beginning. One century later, it was used as tú. Those dignataries, who were very pleased when somebody used to speak to them with vos, at that moment, they felt offended by the use of vos.
How "usted" arised in Spanish
Other forms were needed to express courtesy. That’s how vuestra merced, vuestra señoría or vuestra excelencia were used to replace vos. Only one of them had success: vuestra merced. The continuous use of these forms made them have a shortened form: usted (vuestra merced), usía (vuestra señoría) or vuecencia (vuestra excelencia). And this is how we still use usted in modern Spanish.
The personal pronouns in America
During the conquest, the form vos arrived very eroded to America. In Mexico, Peru and part of Colombia the situation was similar to Spain. They accepted the new form usted.
On the other hand, in the Southern countries (Argentina, Chile, Uruguay, Paraguay…) there are two different scenes: the use of the three pronouns (vos, tú and usted) or only two (vos and usted). In countries when they use the three forms, vos is always the most familiar and colloquial.
What happened with "vosotros" and "ustedes"
In some areas in Spain (Andalucia and Canary Islands) and most of America, the speakers don’t use the second plural person (vosotros). Instead, they use the form of courtesy ustedes, that lost the formal meaning as it happened with vos.
To sum up, the personal pronouns in Spanish are always changing and the evolution is cyclic: one form loses their original meaning because of the constant use and another form arises. Nowadays, the form usted is degenerating in Spain and eventually, it will disappear.