I dont understand this part. Does it mean we don't use un/une/les when the sentence is negative? 

Whereas partitive articles du, de la, de l', des and indefinite articles un, une become de or d' in negative sentences [See Du, de la, de l', des all become de or d' in negative sentences (partitive articles) and Un, une become de or d' in negative sentences (indefinite articles)], this rule doesn't apply to definite articles le, la, l' or les which remain the same in negative sentences