When verbs and expressions must be followed by the preposition de (see list of verbs with de), the de vs du, de la, des rules are similar to those for the descriptive vs possessive de.

1. When de introduces an unmodified noun, use de.

   Il faut changer de train ici.
   You have to change trains here.

   Nous avons besoin d'argent.
   We need money.

2. If the noun that follows de is modified and refers to a specific noun (the word "the" may be needed in English), use de + definite article

   Nous voulons profiter des nouveaux accords.

   We want to take advantage of the new agreements.

   Il s'agit de la réforme agraire.
   It's a question of land reform.

3. If the noun is preceded by an adjective but does not refer to a specific noun (the word "the" is not needed), use only de.

   J'ai besoin de nouvelles chaussures.
   I need new shoes.

   Il se moque de mauvaises idées.
   He makes fun of bad ideas.

Test on de vs du, de la, des