Learn Italian: Mi sono piaciuti i portaombrelli di metallo.

Portaombrelli is a compound noun made of a verb (porta, meaning "holds") and a plural noun (ombrelli, meaning "umbrellas"). When a compound noun is formed by a verb and a noun that is already plural, the compound word is invariable. This means it doesn't change form in the plural; you simply change the article from il to i to show there is more than one.

Why is the verb sono piaciuti in the plural if the English translation is "I liked"?

The verb piacere works backwards compared to English. Literally, this sentence means "The metal umbrella stands were pleasing to me." Because the true subject of the sentence is the plural i portaombrelli, the verb must be in the plural form (sono).

Why does piaciuti end in -i?

Because piacere always uses the helping verb essere in the past tense, its past participle must agree in gender and number with the subject doing the pleasing. Since i portaombrelli is a masculine plural noun, the participle takes the masculine plural ending -i.

Why use di metallo here instead of an adjective?