It's an equivalent of English "a, an". It appears before nouns. Like "a", it indicates that the items or persons are unknown, undefined, or belong to some class.
Forms:
| singular, masculine: | |
| before a consonant: | un |
| before a vowel: | un |
| before gn, ps, z, x and s + consonant: | uno |
| singular, feminine: | |
| before a consonant: | una |
| before a vowel: | un ’ |