DF Blanc: Unité 4, leçon 15 - direct object pronouns le, la, l' les.
Restate the sentence using LE, LA, L', or LES as appropriate instead of the noun as direct object. Then turn flashcard over to check your answer. If you don't understand the answer, just see what part of the first sentence is not on the second. That is what the pronoun le, la, etc. substitutes for.