"Des" becomes " de/d' " in front of adjectives preceding nouns (French Partitive Articles)

Look at these sentences:

Note that when the adjective is placed BEFORE a plural noun, the partitive article des (some) becomes de (or d' in front of a vowel or mute h). 

ATTENTION:
This rule doesn't apply when des is the contraction of "de + les" (= of/from/to the) :

BUT

Not sure which adjectives tend to go BEFORE the noun?

Then see Position of French Adjectives - Short and common adjectives that go BEFORE nouns and Position of French Adjectives - More adjectives that go BEFORE nouns