The Perfect Tense with Regular Verbs

Here the auxiliary verb is avoir. For regular -er verbs, the past participle is formed by replacing the -er ending of the infinitive with -é.
e.g. travailler > travaillé:

j'ai travaillé = I worked
tu as travaillé = you worked
il a travaillé = he worked
nous avons travaillé = we worked
vous avez travaillé = you worked
ils ont travaillé = they worked

Notice that the past participle does not change.
It is the auxiliary verb that changes.