The three main German auxiliary verbs are sein (to be), haben (to have) and werden (to become). These verbs are sort of “helping verbs” that can help to provide the mood or the context of a certain sentence.
Correct is “letztes Jahr sind wir an den Strand gegangen.“
It has to do with the “wir” and the past of go “gegangen”. If the sentence had been with “ich” instead you would have used bin. Hatten and haben both don’t fit to the verb.
