A past participle (el participio) is a very useful form of a verb which can function as an adjective, a predicative, or as a verb in conjunction with haber to form the perfect tenses.
Regular Past Participle Forms
To form the past participle of regular verbs, drop the infinitive ending (-ar, -er, -ir) and add -ado to the stem of -ar verbs and -ido to the stem of -er and -ir verbs. This is equivalent to adding -ed to many verbs in English.
1. The Perfect Tenses
This is the most common use of the past participle and describes what a person has done in most cases.
Perfect Tense Formula
haber + past participle
¿Has viajado mucho a Uruguay? (Have you traveled a lot to Uruguay?)
Habré leído todos sus libros. (I will have read all of his books.)
2. As an Adjective
Past participles that are used as adjectives must agree in gender and number to the nouns they modify.
Me encantan los huevos revueltos. (I love scrambled eggs.)
Salimos por la puerta abierta. (We left through the open door.)
3. As a Predicative Adjective
When used in conjunction with a linking verb (ser, estar, parecer), past participles act as predicative adjectives. Functioning as adjectives, they must agree with the nouns they modify in gender and number.
Mi novela favorita fue escrita por Gabriel García Márquez. (My favorite novel was written by Gabriel García Márquez.)
Las puertas están cerradas. (The doors are closed.)
Esta manzana parece podrida. (This apple seems rotten.)