The past participle is used in two ways in español.  It can be used as an adjective,  just like English. (The cookies were eaten [by Santa]. Las galletas fueron comidas [por el Papá Noel]. )  If it is used as an adjective, remember it must agree with what it is describing.  Also like English it can be used in “perfect” tenses (to say what has, had,  would have, will have, etc. happened).  (I have already eaten.  Ya he comido.) In these instances, there is only one form of the verb- it is not an adjective and does not change form.

Regular verbs:  For most verbs you drop the infinitive ending and add the appropriate ending.  There are no stem changes, work from the infinitive.
 

• Verbos en AR:  ADO  hablar> HABLADO; contar> CONTADO;
• Verbos en ER/IR:  IDO comer>COMIDO, perder> PERDIDO, servir>SERVIDO

Verbs that need accents, so as not to create dipthongs:
 

• EER, AER, OIR: ído- leído, caído, oído, traído, etc.

Irregular verbs.  This list you must memorize.  Or else you fail.
 
 

• Abrir> abierto
• Cubrir> cubierto
• Decir> dicho
• Escribir> escrito
• Hacer> hecho
• Morir> muerto
• Poner> puesto
• Romper> roto
• Ver> visto
• Volver> vuelto

As always, compound verbs will be formed as is the root verb:
 
 

• Maldecir> maldicho
• Descubrir> descubierto
• Describir> descrito
• Satisfacer> satisfecho
• Envolver> envuelto
• Componer> compuesto

Etc.

This is all you need to know about the formation of the past participle.

It is used in compound tenses called "perfect" tenses.  These are formed by using the verb "HABER" followed by the past participle. In these tenses, the past participle is never changed.  The use here is identical to English.

ejemplos

• Ya he comido.  I have already eaten.
• Ellos ya habían salido.  They had already left.
• Espero que hayan comido.  I hope they have eaten.

The past participle is also used frequently as an adjective.  El chico está aburrido.  The boy is bored.  La puerta está abierta.  The door is open.  In these instances, because it serves as an adjective, it must agree with the noun it describes.  The past participle, as an adjective, is employed in many passive voice sentences also.  El Nuevo Mundo fue descubierto por Colón en 1492. The New World was discovered by Columbus  in 1492.   Las sillas fueron compradas en Sevilla. The chairs were bought in Seville.