In English we use the words some or any or no article with nouns that cannot be counted, such as milk,
coffee and tea. In French the partitive is used with these nouns and the partitive article cannot be omitted.
The partitive is expressed by de plus the definite article. Note that de plus le = du and de plus les = des.
The partitive articles are:
  Singular Plural
Feminine de la des
Masculine du des
Masculine or feminin before a vowel de l' des

Study the following:
J'ai de la soupe. I have some soup.
Elle veut du sucre. She wants some sugar.
Il boit de l'eau. He is drinking (some) water.
Voulez-vous des oranges? Do you want any oranges?
Nous avons des livres. We have some books.

The definite article is used with nouns in a general sense. The partitive is used with an undetermined
quantity of a noncountable item.
Il aime le café. (all coffee) He likes coffee.
Il boit du café. (some coffee) He drinks (some) coffee.
With nouns that can be counted, such as bananas (one banana), or items used in the plural (some
bananas), the indefinite article is used.
Je voudrais une poire.
Je voudrais acheter des tomates et des bananes.
Certain nouns can be count or noncount nouns depending on the way in which they are used. The
definite, indefinite or partitive article can be used depending on the meaning.
Voici le gâteau. Here is the cake. (the cake I bought yesterday)
Voici un gâteau. Here is a cake. (a whole cake).
Voici du gâteau. Here is some cake. (part of the cake, a piece of cake)

Exceptions to the rules | Practice with Partitive Article (full and minimal) versus Definite Article