Hi Inma
In the lesson you say:
"There is no difference in meaning. Using one or the other depends on what the person is used to; some regions in Spain use one form more than the other, but both are correct."
However, I always believed their is difference that exists between the two:
Olvidar = to forget by accident
1) Siempre olvido las llaves del coche = I always forget my car keys
2) ¡Ay no, olvidé hacer los deberes! = Oh no, I forgot to do the homework! (olvidar+infinitive)
Olvidarse de = to forget by choice, on purpose.
1) Ella se olvidó de sus problemas de dinero y se fue de compras = She forgot about her money problems and went shopping anyway (she chose to forget)
2) Solo quiero olvidarme de arruinar la fiesta de cumpleaños de mi hermano = I just want to forget about ruining my brother's birthday party (he's trying to forget)
I've always thought of it like the 'se' is indicating that someone is doing something to his/herself (like ducharse, bañarse etc). I'm unsure where I learned this!
The accidental 'se' part is new to me
Saludos