While most Italian nouns change their ending in the plural, a special group of feminine nouns ending in -i do not. These nouns—often of Greek origin and used in academic or scientific contexts—are invariable, meaning their spelling stays exactly the same in both singular and plural forms.
Because the noun itself doesn't change, you must rely on articles (like la, le, or l') and adjectives to determine the number.
For example, l'analisi (the analysis) takes l' in the singular but the standard feminine plural le in the plural:
| The scientist has just finished the chemical analysis. | Lo scienziato ha appena finito l'analisi chimica. |
| We had repeated the chemical analyses in the lab. | Avevamo ripetuto le analisi chimiche in laboratorio. |
Similarly, with words like la crisi (the crisis), only the surrounding words change:
| We had a serious problem because of the crisis. | Avevamo un problema grave a causa della crisi. |
| The scientific project has overcome the crises. | Il progetto scientifico ha superato le crisi. |
Common Invariable Nouns
Many invariable feminine nouns share the same -si ending, such as la tesi (thesis), la sintesi (synthesis), and la diagnosi (diagnosis).
| The student had written a long thesis. | Lo studente aveva scritto una tesi lunga. |
| The patients' diagnoses are not always exact. | Le diagnosi dei pazienti non sono sempre esatte. |
Inside the Laboratory
When discussing scientific research, you will also use regular nouns like il campione (the sample) and il microscopio (the microscope).
| We observed the sample under the microscope. | Abbiamo osservato il campione al microscopio. |
| The chemical reaction has ruined the sample. | La reazione chimica ha rovinato il campione. |
