4-1 Indefinite Article After Expressing Negation

• An indefinite article before a noun (I'll say), weather singular or plural, in a previously would be positive statement usually
  
• becomes de or d' after using n'... pas/expressing in the negative
  

J'ai un cours difficile. > Je n'ai pas de cours difficile.
I have a difficult class. > I do not have a difficult class.