The full table
Pick the article from the noun’s gender and first sound. There’s no plural: “a” is always one thing.
| m · most nouns | un libro, un caffè |
| m · s + consonant, z, gn, ps | uno zaino, uno studente |
| m · vowel | un amico (no apostrophe) |
| f · most nouns | una casa, una stazione |
| f · vowel | un’amica, un’arancia |
Un’ only ever comes before a feminine noun. Un’amico is always a mistake.
Feminine nouns never take uno: una zucchina, una studentessa.