This is the 5460th most frequent Spanish word.
olvidarse
To forget.
In this sentence, 'olvidarse' is used reflexively (indicated by 'se') to mean that the person inadvertently left behind the keys.
Se me olvidaron las llaves en la casa.
I forgot the keys at home.
Here, 'olvidarse' is used reflexively to express the act of failing to recall or remember in a proactive intent.
No quiero olvidarme de tu cumpleaños.
I don't want to forget your birthday.
In this example, 'olvidarse' is used reflexively in the present perfect tense to inquire about an instance of forgetting an event.