This classification:
Accidental/No Fault ‘SE’
The Obligatory Pronominal
For Adding Nuance or Aspect Change
might be an oversimplification of the matter and seems to have been taken from a Spanish grammar for non-Spanish speakers, but I'll try to provide an explanation along those lines.
- Here, only "se" (third person) can be used to express impersonality. As Teusdu said in the comment above, in a sentence like:
- Se rompió. (It broke)
or
- Se rompieron. (They broke)
nobody is taking responsibility for the incident. The verb "romperse" is intransitive and is used instead of the transitive "romper" to avoid mentioning who broke the object(s) in question.
Within this usage, we can also include the "se"-passive, the only difference being that, in this case, a transitive verb is used (only transitive verbs can be turned into the passive voice). The sentence:
- Se vendió el cuadro. (The painting was sold)
is equivalent to:
"Vender" (sell) is transitive, and the passive is used to, once again, avoid mentioning the subject.
- "Se" and similar pronouns for other grammatical persons (me, te, nos) are used with some intransitive verbs. Here, the pronominal particle is necessary for the verb to be intransitive (that's why they call it "obligatory"). In this usage, verbs used to be called "quasi-reflexive" (in current grammar, they are called pronominal), for example:
- Se lastimó. (He got hurt)
They can also be used to express reflexivity. Actually, the sentence above can be used to refer to somebody who injured themselves on purpose:
- Se lastimó. (He hurt himself)
In the case above, the verb is transitive but the person injured (object) is the same as the person who inflicted the injury (subject).
Another case is that of reciprocity, where the verb is again transitive:
- Se aman. (They love each other)
- "Se" and similar pronouns for other grammatical persons (me, te, nos) are used with some transitive and intransitive verbs to express interest, possession, completion, etc. In this case, the pronouns are not obligatory (the grammarian Andrés Bello called them "superfluous") and are only added to express some additional meaning, for example:
Se comió toda la comida. (He ate up all the food) (We can also say "Comió toda la comida", but this lacks the emphasis on completion).
Se come las uñas. (He bites his nails: meaning of possession)
To express interest, we will always find the pronouns under (2) above (i.e. forming pronominal verbs) combined with other pronouns, with the latter referring to the person affected by the action, that is, the person whose interest is involved:
- No se me acercó. ("se" corresponds to the pronominal verb "acercarse" -- approach, get near -- and "me" means "to me"=> He didn't approach me)