What is the rule for conjugating verbs in the vos form in the present subjunctive? If it varies by region, what are the differences?
The Diccionario panhispánico de dudas explains that there are two major types of voseo:
The Diccionario panhispánico de dudas provides a detailed explanation of the regional variants (in Spanish) and includes a voseo conjugation table by country and tense for the verbs cantar, comer, and vivir . Wikipedia explains the regional differences in the use of voseo under the present subjunctive for the irregular verbs dormir and mentir:
There aren't any differences between the
After that, the
It is generally conjugated as the second person, or like the 'vosotros' conjugation (second plural), but wihout the last i.
Second person: (tu) tengas Second plural: (vosotros) tengáis
Second person: (tu) pusieras Second plural: (vosotros) pusierais
As pointed by Alpha, the conjugation is related to the second plural ('vosotros') (*). The general rule is to trim the last 'i'
tú comes (you eat) (second singular, standard form)
vosotros coméis (you eat) (second plural)
vos comés (you eat) (second singular, with 'voseo')
In the imperative, the final 'd' is suppresed
eh, tú, espera (hey, you, wait)
eh, vosotros, esperad
eh, vos, esperá
And, as always, there are some irregularities, eg
" tú, dime" "vosotros, decidme" "vos, decime" (hey, you, tell me)
(*) To make things more confusing, 'vosotros' is not much used in some regions with 'voseo', as Argentina; here, we mostly use 'ustedes' as second plural pronoun, plural form of 'usted' (respectful second person). Both 'usted'/'ustedes' are conjugated as third person.