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Feminine nouns ending in '-i' and '-ie'

In Italian, making a noun plural usually means changing its final vowel. However, there is a specific group of feminine nouns ending in -i and -ie that completely ignores this rule. They look exactly the same whether you are talking about one of them or one hundred.

For English speakers, this can feel a bit like words such as sheep or aircraft, which also stay the same in the plural. In Italian, you will often encounter words like crisi or serie that follow this pattern. To know if these words are singular or plural, you have to look at the words around them.

Word list
eclissifB1 eclipse un'eclissi / le eclissi seriefB1 sequence una serie / le serie analisifB2 analysis un'analisi / due analisi crisifB2 crisis la crisi / le crisi eclissi lunarefB2 lunar eclipse un'eclissi lunare / le eclissi lunari eclissi solarefB2 solar eclipse un'eclissi solare / le eclissi solari speciefB2 species una specie / diverse specie tesifB2 thesis la tesi / le tesi analisi dei datifC1 data analysis un'analisi dei dati / le analisi dei dati cariefC1 cavity una carie / due carie diagnosifC1 diagnosis la diagnosi / le diagnosi fotosintesifC1 photosynthesis la fotosintesi / le fotosintesi (plural rare, form unchanged) ipnosifC1 hypnosis l'ipnosi / le ipnosi metropolifC1 metropolis la metropoli / le metropoli miniseriefC1 miniseries la miniserie / le miniserie parentesifC1 parenthesis una parentesi / due parentesi protesifC1 dentures la protesi / le protesi protesi dentariafC1 dentures la protesi dentaria / le protesi dentarie crisi di nervifC2 nervous breakdown una crisi di nervi / due crisi di nervi crisi finanziariafC2 financial crisis la crisi finanziaria / le crisi finanziarie ipotesifC2 hypothesis un'ipotesi / due ipotesi specie in pericolofC2 endangered species una specie in pericolo / le specie in pericolo specie in via di estinzionefC2 endangered species una specie in via di estinzione / molte specie in via di estinzione specie minacciatafC2 endangered species una specie minacciata / le specie minacciate
Looking for plural clues

Because nouns like analisi, crisi, diagnosi, and metropoli never change their endings, the job of showing the plural falls entirely on articles, adjectives, and verbs.

When you read or listen to Italian, you must spot the definite article (like le instead of la) or the adjective ending (like nuove instead of nuova) to know how many things are being discussed.

La nuova analisi rivela un problema.
(The new analysis reveals a problem.)
Le due analisi confermano la diagnosi.
(The two analyses confirm the diagnosis.)
Hanno superato una crisi difficile.
(They overcame a difficult crisis.)
Queste crisi offrono anche nuove opportunità.
(These crises also offer new opportunities.)
Ho letto la sua tesi di laurea.
(I read her university thesis.)
Words ending in -ie

The same rule applies to a small but common group of feminine nouns ending in -ie. Words like serie, miniserie, specie, and carie do not change in the plural.

Ho guardato una serie televisiva fantastica.
(I watched a fantastic television series.)
Il dentista ha curato due carie.
(The dentist treated two cavities.)
Abbiamo scoperto tre nuove specie di uccelli.
(We discovered three new bird species.)
Adjectives still change

You need to pay special attention when these nouns that do not change are part of a descriptive phrase, such as crisi finanziaria or eclissi solare. While the noun itself stays frozen in the plural, the adjective attached to it must change to match.

For example, finanziaria becomes finanziarie, and minacciata becomes minacciate.

Le crisi finanziarie colpiscono molti paesi.
(Financial crises affect many countries.)
Dobbiamo proteggere le specie minacciate.
(We must protect threatened species.)
Le eclissi solari sono eventi molto rari.
(Solar eclipses are very rare events.)
Summary

When using feminine Italian nouns that end in -i or -ie, remember that the noun itself does not change in the plural. You must use plural articles and plural adjectives around the noun to show that there is more than one.