You've already seen how to use indefinite articles un or une to express a/an (see Using un, une to say "a" (indefinite articles))
Now look at these negative sentences:
Indefinite articles un and une become de or d' (in front of a vowel or mute h) after a negative expression (ne...pas / ne...jamais / ne...plus ... etc.) in order to express no / any.
ATTENTION:
This rule does NOT apply to sentences using the verb être and other Verbes d'état, with which the indefinite article doesn't change:
EXCEPTION:
When you want to emphasise the meaning of ONE (un/une) - not just a/an - as in He doesn't have ONE car, but TWO, you will keep un/une in the negative sentence - but here it doesn't mean no/any:
Also see Du, de la, de l', des all become de or d' in negative sentences (partitive articles)
Note that definite articles (le, la, l', les) don't change in negative sentences:
J'aime le chocolat. -> Je n'aime pas le chocolat.
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