it's "the personal a"; read about it here: http://spanish.about.com/cs/grammar/a/personal_a.htm
thanks. I was not actually caring about the conjugation of the verb, just curious if there was another verb / synonym used as well in this context of remembering people. Could there be differences too for example if you 'remembered who someone was after not seeing them for a long time', or you 'remembered someone in your will', using remembered more as thinking of someone and being thoughtful towards them in your actions?
Feliz ano nuevo :)
Ah, sorry misread what you wanted then.
The verb you want to know for those uses is recordar, which is the verb of remembering, recalling, and of memory:
La vida sería imposible si todo se recordase. El secreto está en saber elegir lo que debe olvidarse.: "Life would be impossible were everything remembered. The secret is in knowing what should be forgotten"
El recuerdo de aquella noche "The memory of/Remembering that night"
Recuerdos de una noche: "Memories of that night"
The related phrase, acordarse de, is also useful and is usually the same in meaning, but it is slightly less formal and can't be used to describe the action of being reminded.
I believe it is an exercise to use 'usted' (2nd person formal but uses 3rd person to translate, in the past tense.) However If one had a translation sentence from English to Spanish there would not be any context so, one could use either the 2nd person familiar which is as you suggested 'recordiste' or you could use the formal second person usted which is recordó.
Thanks jfgordy, I appreciate your explanation. However, it is an exercise on the use of the past tense and not on the use of "usted". I can see by your explanation that I can interchange the 3rd person singular by the formal second person singular depending on the context. I did get my answer, only because of the context (like you suggested). Interesting! Thanks again.