Question: I might be studying in Argentina for a semester. I have heard that they use a vos form instead of tú, and I wondered if you could tell me how that is conjugated. Also, do you know how that different form developed? I took a semester of Portuguese and I wondered if it's at all related to their familiar "you" form, voce.
Answer: I hope your studies in Argentina work out. It's an interesting country to visit, with a rich history and culture.
However, as you already are aware, Argentine Spanish does have its differences from the Spanish of other areas. Perhaps the most well-known to outsiders is its pronunciation. There's nothing you won't be able to understand or get used to, as the differences are probably no greater than between Australian English and U.S. English. For example, you may hear the ll receive the "zh" sound, like the "s" in "measure" or the "z" in "azure."
Not so well known is the major grammatical difference, the use of vos instead of the far more common tú as the second-person familiar subjective pronoun. Although vos is heard elsewhere as an alternative to tú, or among certain groups of people, in Argentina it basically replaces tú in everyday speech among people of all classes and education levels.
I don't know the specifics of how vos developed in Argentina, but it is fairly safe to assume that it is the form that was used by the early Spanish settlers in that country. As Spanish overtook the indigenous languages of the country, people adopted the Spanish of those settlers. As this occurred in the days before electronic media, there would have been little outside influence that would have acted to make Argentine Spanish more consistent with that of other countries.
Although it hasn't happened in the case of Argentina, in the course of time groups of people who start off speaking one language can each develop their own variations, leading to the formation of new languages. Originally, of course, the two languages you mention, Spanish and Portuguese, started out as Latin. Thus the Portuguese voce and Argentine Spanish vos indeed have a common origin — along with the French vous and Italian voi. It may interest you to know that tú has an etymological connection with the archaic English familiar form "thou," as both the Romance and Germanic languages themselves developed from Indo-European. French and German have similar words, tu and du, respectively.
In some places where vos is used, it takes the same verb forms as does tú. But not so in most of Argentina. Generally speaking, present-tense verbs take the endings of ás added to the root of -ar verbs, és for -er verbs, and í for -ir verbs. And because the accent is on the final syllable, you won't find the stem changes that you do when tú is used. The present-tense, second-person familiar form of tener (to have), for example, is tenés, and the present-tense form of poder is podés. Among the irregular forms is sos for ser. Thus, vos sos mi amigo is the equivalent of tú eres mi amigo, or "you are my friend."
See the following page for details on how vos is used in Guatemala.