Which indefinite article (un, una)? 2

Un/una with modified vs unmodified nouns — After ser, adding an adjective to a profession brings the indefinite article back: Soy médico (no article — unmodified) vs Soy un médico muy ocupado (article returns with adjective). The same rule applies to other nouns of category: Es problema grave → Es un problema muy grave.

Otro, tal, cierto, semejante — no article — A few adjectives in Spanish push the indefinite article out: otro libro (another book — not un otro libro), tal persona (such a person), cierta actitud (a certain attitude), semejante idea (such an idea). Never place un/una before these adjectives.

The article in comparisons and exclamations — ¡Qué + noun (exclamation) takes no article: ¡Qué libro!. But ¡Qué + adjective + noun does: ¡Qué un libro tan interesante! (formal/literary). In everyday speech, the article is usually dropped in exclamations. For comparisons: tan + adjective + como never needs an article.