Translation:I wonder what he remembered to do.
I am totally confused here. If 'si sia ricordato' is past subjunctive, how do you know whether it refers to first, second or third person. 'I remembered' doesn't fit the context of the rest of the sentence, but 'you remembered' does. How can you differentiate? Or have I got this completely wrong. HELP!
Sia is the singular person subjunctive mood of the helping verb essere which pairs up with the past participle ricordato. The helping verb is a form of essere (not avere) due to the reflexive ricardarsi.
So the literal translation is:
I ask myself = mi chiedo,
what = cosa,
he recalled to himself (with subjunctive due to the uncertainty of what he remembered to do) = si sia ricordato,
to do = di fare.