When do you use "être" and when "avoir" to form the past perfect in French?

The general rule is that "être" is used as the auxiliary verb in the past tense for verbs of movement and which are intransitive, i.e. cannot be followed by an object without a preposition: "je suis allé en Grande-Bretagne", "nous sommes tombés de l'arbre", "elle est sortie de la maison". Remember that the past participle must be accorded to the subject in these constructions: "nous sommes tombés", "elle est sortie". All other verbs take "avoir", even verbs of movement: "j'ai mangé une pomme", "ils ont grimpé la montagne", "vous avez parlé de la France".Be careful about reflexive verbs, which always require "être" in the past tense: "je me suis lavé", "nous nous sommes amusés", "il s'est moqué".