grammar.articles.indefinite-uses
When un/una disappears: soy ingeniero
No article with professions, nationality, or religion after ser (es médica), after sin/con, and before otro, medio, cien, mil: otro café, media hora.
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Explicación
Spanish drops the indefinite article where English keeps 'a/an'. The classic case is an unmodified profession, nationality, or religion after ser: es médica, soy ingeniero, es boliviano, es católico. The article returns as soon as you add an adjective: es una médica excelente, es un ingeniero joven.
It also disappears after sin and con (sin coche, con paciencia, sin idea) and in many tener phrases about categories (tengo coche, no tengo hermanos, tienen casa propia).
And it never appears before otro, medio, cierto, tal, cien, mil: dame otro ('give me another', never *un otro — a French/English calque), media hora, cien personas, mil veces, tal cosa.
Ejemplos
My partner is an architect.
Región: global
Can you give me another napkin?
Región: global
My sister is a teacher and doesn't have a car yet.