Most learners say "Me olvidé las llaves." That works. But listen to a native speaker who just locked themselves out: "Se me olvidaron las llaves." Look at the verb. Olvidaron is plural, and that's the whole trick. The subject isn't me anymore, it's the keys. I'm just the person it happened to, that little "me" in the middle. Se me olvidaron las llaves. = My keys slipped my mind. So the grammar quietly moves the blame. Nobody did anything, it just happened, and I'm the one stuck outside. Same thing with se me cayó el móvil or se me rompió el vaso.