As we have seen before (Advanced 17, Related Grammar), past participles can be used as adjectives. Thus, they must follow the rules of all adjectives:

1. Spanish adjectives often have four distinct forms: masculine singular, feminine singular, masculine plural and feminine plural.

2. Adjectives that end in -o in the masculine singular form have a feminine form which ends in -a. The plural is formed by adding an s to each singular form(e.g. pequeño, pequeña, pequeños, pequeñas).

3. Adjectives whose masculine singular form does not end in -o tend to have the same form for both genders. The plural is formed by adding an -s to the singular if it ends in a vowel and -es if it ends in a consonant. If it ends in a -z, the plural form will change to a -ces. In the following list the first row is for singular, the second is for plural forms of the adjective.

	Masculine	Feminine
	interesante	interesante
	interesantes	interesantes
	útil	útil
	útiles	útiles
	feliz	feliz
	felices	felices

	Estas velas blancas son muy caras.	These white candles are expensive.
	Los bancos están abiertos.	The banks are open.

As we can see in the last example, the rules for adjectives apply to past participles when they are used in the estar + past participle formation. They also apply when a past participle is used as an adjective without estar (e.g. el estudiante preocupado). However, when a past participle is used as a verb in one of the perfect tenses, with the construction haber + past participle, it does not follow the rules of adjectives. Compare, for example, the following sentences:

	Se ha cerrado la puerta.	The door has been closed.
	La puerta está cerrada.	The door is closed.
	He prendido las luces.	I have turned the lights on.
	Las luces están prendidas.	The lights are turned on.
	Hemos congelado la carne.	We have frozen the meat.
	La carne está congelada.	The meat is frozen.