The perfect tense

. The participle is used for an action that has been completed. It expresses that something has happened at some point before now, at an unspecified time in the past. It can also indicate that the action is still happening.

. It cannot be used with specific times, unless it indicates a repetition of actions during that period of time, for example: aún, todavía.
 

. We need the present form of the auxiliary verb “haber”:
   He
   Has
   Ha
   Hemos
   Habéis
   Han

. In order to form the participle, we need the stem of the infinitive verb and we add “–ado” if the verb ends in –ar or “–ido” if the verb ends in –er or –ir.

  Comprar – comprado

  Comer – comido
  Vivir – vivido
 

. Here you have a list of irregular participles:

  Abrir – abierto
  Caer – caído
  Cubrir – cubierto
  Decir – dicho
  Escribir – escrito
  Hacer – hecho 
  Leer – leído
  Ir – ido
  Morir – muerto
  Poner – puesto
  Romper – roto
  Ser – sido
  Traer – traído
  Ver – visto
  Volver – vuelto