Gary Kinghorn alerted me to the Argentine use of vos rather than tu for the 2nd person familiar/intimate pronoun and the surprising verb conjugations that go with it.
Here's Gerald Erichsen's brief explanation of the verb forms from About.com.
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."
