The difference is a little like 'some' and 'any' in English. Affirmative statement: Les femmes ont du riz = The women have some rice Here you have a positive verb form (ont) followed by 'du', meaning 'some' Negative statement: Les femmes n'ont pas de riz = The women don't have any rice Here you have a negative verb form (n'ont pas). After a negative verb form, you can't use any of the words meaning 'some'. You have to use 'de' instead. Together with the negative verb, 'de' indicates a lack of something, the same as 'not + any'. I hope that helps.