test items pertaining to this lesson

Question

Hi Inma,

The test questions given at the end of this
section are as follows:

1.       ________ poner el despertador. He [accidentally] forgot to
set the alarm clock.HINT: Use a construction with the verb "olvidar" that expresses
an "unintentional action"

Correct answer given: Se
me ha olvidado

2.      
________ poner el despertador.He [accidentally] forgot to set the alarm clock.HINT: Use a construction with the verb "olvidar" that expresses
an "unintentional action"

Correct
answer given: Se le olvidó

Both examples feature an action represented by a verb that
follows a form of olvidar. This is apparently an example of the accidental “se”.
But the examples in this lesson use a different structure. The object forgotten
is the subject of the sentence and olvidarse is conjugated with that in mind. It
is explicitly stated that the verb is conjugated in the third person plural “to
agree with the subject .”  That is not
the case on either test item. Plus the test “hints” given are not useful for
two reasons:  1) They ask that you use
the verb “olvidar” instead of “olvidarse” (misleading) and 2) it says that the
verb expresses an “unintentional action”; which is true of all forgetting , no?
However, I think I could have figured out what was wanted If the sentence
structure of the test item were not different from the lesson (e.g. in the
lesson the thing forgotten was the subject of the verb).

Answer

Hola Wylie,
I am not quite following, sorry. The example that you are highlighting from the test is one that needs the verb in the 3rd person singular. What I am realising is that we need to add extra information to the lesson saying that when it is expressing "Someone forgot to do something" you need to conjugate in the 3rd person singular as if "to do" was the subject, and the infinitive is taken as a singular thing. 
For example:

Se me olvidó comprar pan. ( comprar pan; pan is singular )
Se me olvidó comprar cervezas. (comprar cervezas; cervezas is plural, but olvidar is still in the 3rd p.sing, agreeing with "comprar", not with the plural subject "cervezas")

I don't think that is reflected in the lesson, so I will add it.
In terms of the "unintentional meaning" of the verb olvidar, you are right, it is an implicit meaning of this verb. But using this specific structure with "se" is what is called in grammar "the accidental se". The hint is simply indicating that you need to use that structure instead of the more direct "olvidar": "Olvidó las llaves" or "Se olvidó de las llaves". 
The accidental se is applied to lots of other verbs. It is called "accidental" to detach the person from the guilt/responsibility of whatever happens. 
I hope this clarified it.
Saludos
Inma