Is comerse required when used with a determinate amount?

Question

I have already read some material about the difference between the verbs "comer" and "comerse":

• ¿Cuál es la diferencia entre comer y comerse?
• El uso de "comerse"
• https://forum.wordreference.com/threads/comer-comerse.2173683/
• https://forum.duolingo.com/comment/27359275/Comer-vs-comerse

If I understood right, "comer" is neutral and "comerse" is used to express emphasis on the completeness of the action of eating (as "eat up"). In some contexts, it may also imply voracity or delight in the action of eating.

The confusing thing is that some people state that "comerse" is only used and it is the only correct verb when there is a determinate amount of the thing eaten (eg la manzana, esta manzana, una manzana, dos manzanas, etc). Is it indeed the only correct option or is it just usual to use it whenever there is a determinate amount ? If it is the latter, is it indeed usual in all countries which speak Spanish or is it a regional thing? Notice that "eat" and "eat up" have no such restrictions in English.

Examples for discussion:

• Como chocolate todos los días (OK)
• Me como chocolate todos los días (wrong)
• Comí una manzana. (OK or wrong?)
• Me comí una manzana (OK, emphasis on completeness)

It would be helpful if people answering this question mentioned their country in the answer.

Answer 1

Comerse (and other pronominal forms of verbs that imply consumption or the taking of possession) is indeed used as you have gathered from those sources. This usage is called aspectual dative (scroll to the last part of the answer).

There are a few instances where comerse is the only grammatically correct form (see below). Leaving aside fixed phrases and some other specific cases, this has to do with whether the object is bounded.

An object is bounded (in this sense) if it's definite (la manzana, esta manzana) or appears as a definite quantity (una manzana, dos manzanas) or expresses totality (toda la manzana, todas las manzanas). For all of those you can use comerse, although comer is correct too, with the differences in connotation that you already know.

• Voy a comerme una manzana.
• Ayer me comí las manzanas que recogí la semana pasada.
• ¿Te comiste toda la manzana? (OR: ...la manzana completa?)

You cannot use comerse when the object is unbounded, i.e. when it's countable but not definitely quantified (manzanas) or when it's uncountable (manzana, as in apple used as an ingredient of something, whether explicit or not). Here you must use comer:

• Ayer comí manzanas. "Yesterday I ate (some) apples."
• Comí pasteles de manzana. "I ate apple pies."
• Ayer comí manzana. "Yesterday I ate apple."
• Comí pastel de manzana. "I ate apple pie."

In your examples the only doubtful one was “Comí una manzana.” This sentence is correct. In my dialect (Rioplatense, from Argentina), this would sound a little off if you were talking to a friend about your last meal. It actually sounds a bit clinical, like what you would tell a doctor. In any case, comerse is overwhelmingly preferred to comer in informal speech in cases when it's allowed.

There are some cases, not obviously derived from the rules above, where comerse is to be used and comer would be wrong. These are (thanks to @Gustavson for pointing these out):

• Fixed phrases: “Me comí todo” (which literally means "I ate everything", but can figuratively mean "I ate a lot, too much, until I couldn't eat no more"); also “Me comí la vida”.
• Figurative expressions: “comerse las eses” ("to eat up one's S's", i.e. s-aspiration); “comerse a alguien” ("to have sex with someone"); "comerse una paliza / un reto" ("to put up with a -- usu. undeserved or excessive -- beating or reprimand").
• Consumption of prey by carnivores: “El león se comió a la gacela” ("The lion ate the gazelle", even if the lion didn't actually ate the whole gazelle).