grammar.se.accidental
Se #2: accidents — se me cayó, se me olvidó
Se + indirect pronoun + verb presents mishaps as happening to you: se me olvidaron las llaves, se le rompió el vaso. The verb agrees with the thing.
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Explicación
The 'accidental' or unplanned se presents a mishap as something that happened to you rather than something you did. The frame is se + indirect-object pronoun + verb + the thing: se me olvidaron las llaves, se le rompió el vaso, se nos acabó el tiempo. Literally, 'the keys forgot themselves on me'.
It quietly removes blame, which is why it's so common: se me cayó el celular ('the phone dropped on me') sounds less culpable than lo dejé caer ('I dropped it'). The me/te/le/nos/les marks who it happened to.
The verb agrees with the thing, not the person: se me olvidó el nombre vs se me olvidaron los nombres. The star verbs: olvidar, caer, perder, romper, acabar, quedar (se me quedó en casa, 'I left it at home'), and ocurrir (se me ocurre una idea, 'an idea occurs to me').
Ejemplos
I forgot your birthday, sorry.
Región: global
We ran out of gas.
Región: global
I lost track of time and left the report at home.