Past Participle in Spanish

In Spanish the past participle has three uses:

1) as a form of the conjugated verb in the active voice, i.e. "yo he hablado" (I have talked)
2) as a form of the conjugated verb in the passive voice, i.e. "fue usado" (it was used)
3) as an adjective

How to form the Past Participle

Regular verbs have three endings: "ar", "er" and "ir".

For verbs ending in "ar", the past participle is formed by adding "ado" to the root for of the verb:

cantar (to sing) → cantado
bailar (to dance) → bailado

For verbs ending in "er" and "ir", the past participle is formed by adding "ido" to the root form of the verb:

comer (to eat) → comido
perder (to lose) → perdido

asistir (to attend or to help) → asistido
advertir (to warn or to notice) → advertido

However, there are some regular verbs that don't follow the general rule, which must be memorized, for example:

abrir (to open) → abierto
escribir (to write) → escrito
imprimir (to print) → impreso

Concerning irregular verbs, of which the Spanish language has many, the only way to conjugate them correctly is to memorize them:

ir (to go) → ido (follows the general rule)
decir (to say) → dicho (doesn't follow the general rule)

How to use the Past Participle

1) As a form of the conjugated verb in the active voice, it is used in perfect tenses with the auxiliary verb "haber" (to have). In this case, the past participle is always the same, without agreeing with gender or number:

He cantado varias veces en ese lugar (I have sung several times at that place.)
Ana ha bailado en teatros de todo el mundo (Ana has danced in theaters all over the world.)
Los niños han viajado a otro país (The children have traveled to another country.)

2) As an adjective. In this case, there must be an agreement with the gender and the number of the noun it refers to:

la mujer amada (the loved woman)
las mujeres amadas (the loved women)
el hombre amado (the loved man)
los hombres amados (the loved men)

3) As a form of the conjugated verb in the passive voice, but with the auxiliary verbs "ser" (if the action is in course) or "estar" (if the action has been completed):

Este libro es usado por muchos estudiantes ->This book is used by many students
La mujer fue asistida por el servicio de emergencia ->The woman was helped by the emergency service.