When we change compound nouns into their plural forms, there are some general guidelines that we can follow for each category. That said, it is always wise to consult a dictionary when using these nouns; there are many exceptions to the general rules outlined here!
NB: There are some notable exceptions to this general rule, namely la mezzaluna --> le mezzelune and la mezzanotte --> le mezzenotti.
If the noun is, however, singular and masculine, then it is pluralized in the compound.
When the noun element is feminine, the plural form is the same as the singular.
Noun + Adjective
In these cases, both words comprising the compound noun are pluralized.| la terracotta ... le terrecotte la cassaforte ... le casseforti il caposaldo ... i capisaldi la cartastraccia ... le cartestracce |
Adjective + Noun
In these cases, the noun is pluralized.| il francobollo ... i francobolli il mezzogiorno ... i mezzogiorni l'altoparlante ... gli altoparlanti il gentiluomo ... i gentiluomini |
NB: There are some notable exceptions to this general rule, namely la mezzaluna --> le mezzelune and la mezzanotte --> le mezzenotti.
Adjective + Adjective
The second adjective is pluralized.| il sordomuto ... i sordomuti il pianoforte ... i pianoforti |
Verb + Noun
If the noun is pluralized already, the compound plural does not change.| il cavatappi ... i cavatappi l'asciugacapelli ... gli asciugacapelli lo stuzzicadenti ... gli stuzzicadenti |
If the noun is, however, singular and masculine, then it is pluralized in the compound.
| il grattacielo ... i grattacieli il passaporto ... i passaporti |
When the noun element is feminine, the plural form is the same as the singular.
| l'aspirapolvere ... gli aspirapolvere il portacenere ... i portacenere il salvagente ... i salvagente |
Verb + Verb and Verb + Adverb / Adverb + Verb
Wherein a compound noun is formed by two verbs or a verb and an adverb, the plural is the same as the singular form.| il saliscendi ... i saliscendi il buttafuori ... i buttafuori il dormiveglia ... i dormiveglia il benestare ... i benestare il viavai ... i viavai |
Preposition + Noun
The plural is formed by pluralizing the noun.| il soprannome ... i soprannomi l'antipasto ... gli antipasti il sottopassaggio ... i sottopassaggi |
Adverb + Adjective
In these cases, the adjective is pluralized.| il sempreverde ... i sempreverdi |