Un/une become de/d' in negative sentences in French (French Indefinite Articles)

You've already seen how to use indefinite articles un or une to express a/an (see Un/une = A or An (French Indefinite Articles))

Using un/une in negative sentences in French

Now look at these negative sentences:

Il a une voiture. - Non, il n'a pas de voiture !He has a car. - No he doesn't have a car!

Tu as un animal de compagnie ? - Non je n'ai pas d'animal de compagnie.Do you have a pet? - No, I don't have a pet.

J'ai un frère mais je n'ai pas de sœur !I have a brother, but I don't have a sister.

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 List of French "state verbs" - "verbes d'état", with which the indefinite article doesn't change:

Je ne suis pas une menteuse !I'm not a liar!

Mon chien n'est pas un labrador.My dog is not a Labrador.

Il reste un ami loyal. - Non, il ne reste pas un ami loyal !He remains a loyal friend. - No, he doesn't remain a loyal friend!

Elle est devenue une excellente danseuse. - Non, elle n'est pas devenue une excellente danseuse !She became a great dancer. - No, she didn't become a great dancer!


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 - here it doesn't mean no/any:

Ils n'ont pas une maison, mais deux !They don't have ONE house, but TWO!

Also see Du/de la/de l'/des all become de/d' in negative sentences (French 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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Examples and resources

J'ai un frère mais je n'ai pas de sœur !I have a brother, but I don't have a sister.
Mon chien n'est pas un labrador.My dog is not a Labrador.
Je ne suis pas une menteuse !I'm not a liar!
Ils n'ont pas une maison, mais deux !They don't have ONE house, but TWO!
Il reste un ami loyal. - Non, il ne reste pas un ami loyal !He remains a loyal friend. - No, he doesn't remain a loyal friend!
Elle est devenue une excellente danseuse. - Non, elle n'est pas devenue une excellente danseuse !She became a great dancer. - No, she didn't become a great dancer!
Il a une voiture. - Non, il n'a pas de voiture !He has a car. - No he doesn't have a car!
Tu as un animal de compagnie ? - Non je n'ai pas d'animal de compagnie.Do you have a pet? - No, I don't have a pet.
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