Past Participles are verbs that have the characteristics of adjectives. These participles change the infinitive ending for: -ado, -ido, -to, -so, -cho. Participles functioning as part of phrasal verbs in the present perfect always end in o. Participles functioning as adjectives agree in gender and number with the noun they are modifying. Let's see a few examples:

decidir (to decide)
decidido (determined)

equivocarse (to make a mistake)
equivocado (wrong)

Some participles have both a regular and irregular form:

recluir (to imprison)
recluido (imprisoned)
recluso (imprisoned)

Example sentences:

La marimba es el instrumento más conocido de Guatemala.
The marimba is the most known instrument of Guatemala.

Nuestros abuelos están muertos.
Our grandparents are dead.

No es un bebé recién nacido.
It isn't a newborn baby.

Necesito ver si está herida.
I need to see if she is hurt.

Estoy tan preocupado que ya no duermo.
I'm so worried that I cannot sleep.