Comistes

Comistes
 

In the varieties that use this form instead of comiste, is this derived from the archaic vos(otros) form comisteis?

I wouldn't say it's derived from the archaic vos, since that conjugation was comites, but you'll hear it once in awhile. Some would say that the less educated use it (but only where el voseo is used). I've heard the final 's' used, but it isn't proper Spanish. You don't need to learn how to use it, you just need to learn how to be tolerant of those who do. ;)

Comites? Are you sure?

http://www.verbix.com/webverbix/Spanish/comer.html
This website says:
Pretérito perfecto simple
yo comí
tú comiste
él comió
nosotros comimos
vosotros comisteis
ellos comieron

That is the current cojugation.
You asked about the archaic usage:
vos comites (which has been replaced by vosotros comisteis)

Oh. I thought the reverential vos forms were identical to the vosotros forms, and preserved in the modern vosotros forms. Is it just that one word that is irregular, or what is the rule to form those forms?

The 'reverential' vos form is archaic. Vosotros, and its current conjugation, is its replacement. This is used in Spain.

The vos form used in some Latin American countries is not the archaic vos form. It has its own conjugation (it's not the same as the current vosotros and it doesn't match the archaic form). Plus, there is more than one conjugation of vos, depending on the country you live in.

There are websites that teach you how the current form of vos is used. I couldn't find very many resources on the archaic form (since it's no longer used).

By archaic vos, I meant the one that used to be used in Spain. It is the same as the vosotros form, right? Don't you think that comistes probably came from comisteis? They're the same except for the absense of the i, and this smoothing of diphthongs seems to be quite common (for instance in some countries modern day use of vos habláis > hablás, for isntance.) So rather than just being "improper Spanish" springing out of thin air, maybe it came from the vos(otros) form and won out over the tú form in certain speakers in informal speech, just like the modern day vos forms tended to displace and replace the tú forms in countries that use them.

There are certainly some theories saying that adding a letter "s" to the end of that conjugation (past tense, second person singular) reminisces from such archaic form, but it doesn't keep any of the original respectful treatment, so nowadays it's just considered an incorrect way to conjugate.

"Comistes" can be:

1) A form for past simple and pronoun "vos" that is current and considered colloquial to vulgar in voseante regions.

2) A mistaken form for past simple and pronoun "tú" that is common among speakers with low education in every Spanish speaking country. It's from low/popular levels and comes from some anomaly in Spanish conjugations:

tú ...

...tienes
...tendrás
...tengas
...tendrías
...tuvieras
...tuvieses
...tuviste

so, there's a sort of 's' as a thematic consonant of second person singular in the mind of many speakers.

3) There are 1) that are treated as 2) in many countries with voseante and tuteante areas, as part of their internal conflicts.

Today, adding an "S" to the second person preterite of indicative: comistes, estudiastes, fuistes, is considered wrong grammar and it gives the idea of being an uneducated person. It sounds terrible to my ears...