Plural indefinite articles before adjectives (de)

In French, the plural indefinite article is usually des (some), used before plural nouns like des mystères (mysteries).

However, when an adjective comes before a plural noun, des changes to de.

Astrology explains great mysteries.
L'astrologie explique de grands mystères.
My horoscope announces beautiful things.
Mon horoscope annonce de belles choses.

Notice that if you remove the adjectives (grands, belles), the sentences revert to using des (des mystères, des choses).

While most French adjectives follow the noun, a specific group—often relating to beauty, age, goodness, and size—comes before it. When using these preceding adjectives in the plural, always use de.

With this telescope, we see new constellations.
Avec ce télescope, on voit de nouvelles constellations.
The horoscope announces bad news.
L'horoscope annonce de mauvaises nouvelles.
This book tells old legends.
Ce livre raconte de vieilles légendes.
The sky shows great omens tonight.
Le ciel montre de grands présages ce soir.