Me olvido de....

Question

In the quiz question A veces ________ que llevamos casados 20 años. I selected the response “se me olvido” which was incorrect — but I’m not sure why, even after reviewing the lesson. I have a couple guesses, but could you help me understand why it doesn’t work?

Accepted Answer

Hola A
To be able to use the structure with "se + indirect object pronoun (me, te, le, nos, os, les), you need the verb agreeing with the subject which is "whatever is forgotten". In this case we have a whole clause as the subject: 
...que llevamos casados 20 años
If we substitute this clause by "esto" to simplify it, we'd say:
A veces se me olvida esto. 
Now we can place the subject at the front so we can see it more clearly:
Esto se me olvida a veces.
You can see how the subject (esto, singular, as if it was he/she) matches with the verb in the 3rd person singular: se olvida
With this structure the verbs won't match with the "person" (yo, tú, María...) because the person is not actually the subject in this type of sentence. 
Have a look at these two sentences:

María se olvidó de las llaves.
A María se le olvidaron las llaves.
María forgot her keys.

Two sentences meaning the same but with different structures and different subjects (on the second, the subject is las llaves) Doesn't it remind you of "gustar"? 
Inma