Compound nouns (nomi composti) are formed by joining two words. Their plural forms depend on the types of words combined.
Verb + Noun = Usually invariable
If a compound is made of a verb and a plural noun, the word itself doesn't change. You only change the article. This applies to everyday items like il portachiavi (key ring), l'apribottiglie (bottle opener), and il portaombrelli (umbrella stand).
Compounds formed with a verb and a singular feminine noun are also usually invariable, such as il guardaroba (guardare + la roba).
Exception: A few verb + singular noun combinations behave like regular masculine nouns and take an -i ending. For example, il salvagente (life jacket) becomes i salvagenti.
Noun + Adjective = Both parts change
When a compound is formed by a noun and an adjective, you usually pluralize both parts. For example, la cassaforte (safe) becomes le casseforti.
Noun + Noun = First part changes
If the compound consists of two nouns where the first is the "head," only the first noun changes. For example, a head waiter is il caposala (head of the room). In the plural, only capo changes, becoming i capisala.
Invariable phrases
Words formed by a preposition/adverb + noun (like il dopocena, after-dinner) or shortened noun phrases (like il centrotavola, centerpiece, from centro della tavola) are completely invariable in the plural.
