How do we know when it's "accidental"?

Question

In the example "A veces ________ que llevamos casados 20 años", why is simple olvido wrong? - when there was another question in another kwizz about forgetting a towel, and olvido was correct. How are we supposed to know if something is accidental or not? I must say that I find this lesson absolutely confusing and no matter what I put in the answer, it always seems to be wrong.

(And I notice that it lists me as C1. I'm not C1, I am still struggling with B2. The Kwizbot doesn't understand the fluctuating nature of how people learn, forget, re-learn, get wrong again, and so on.)

Answer

Hola BarbaraThese questions from this topic are not asking to recognize if it's meant to be accidental or not. The lesson tells you different ways to use the verb olvidar, highlighting one of these ways as having that specific nuance of "being more accidental" (e.g. se me olvidó, se nos ha olvidado..., ) The reason why we include all these different ways is because they don't work the same way grammatically. The verb in the "accidental" one is conjugated agreeing with the thing that is forgotten instead of the person who forgets. In the questions we ask to choose the correct form of olvidar. There are normally 4 options with only one being correct. The other incorrect forms either have the verb conjugated incorrectly or is missing one of the pronouns for example. There can't be any other option as correct other than the one that is set as correct in the system, because they wouldn't be grammatically correct. I hope this clarified it.