Some grammar points
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Nouns, pronouns and articles in Italian can be masculine or feminine
When you learn these words in Italian, you need to learn their gender.
The masculine forms of "a" are un and uno. The feminine forms are una and un'.
Uno student (a male student)
Una studentessa (a female student)
Un treno (a train is masculine in Italian)
Una bicicletta (a bicycle is feminine).
Uno is used before a masculine word beginning with z or with s and a consonant.
Questo (this) is used with masculine words.
Questa (this) is used with feminine words.
Uno and una drop the final "o" before a vowel.
Lei è un'infirmiera.
Infirmiera is a feminine noun so una drops the final "a" before it.
Likewise automobile (car) is feminine so the "a" in una is dropped.
Un'automobile
Similarly numbers ending in vowels (e.g quaranta = 40) drop them before a word starting with a vowel.
Ha quarant'anni.
Ha vent'anni
As Italian verbs are inflected, personal pronouns (lui = "he", lei = "she") are often unnecessary.
Thus, instead of saying "Lui lavora in Roma" Italians will usually drop "lui" and just say "lavora in Roma".