Learn French: Je prépare du poisson avec beaucoup de pommes de terre. - I am preparing some fish with a lot of potatoes.

After expressions of quantity like beaucoup (a lot) or un peu (a little), you always use the simple word de (or d' before a vowel). It does not change to des even if the following noun is plural.

Why do we use du before poisson but only de before pommes de terre?

Du is used here to mean "some" (some fish). However, when you use a specific quantity phrase like beaucoup de, it replaces the "some" word. You use du or des for an unspecified amount, but always de after beaucoup.

Wait, doesn't pomme mean apple?

Yes! The French word for potato is pomme de terre, which literally translates to "apple of the earth." Notice that when making it plural for multiple potatoes, only the first word gets an "s" added: pommes de terre.

The translation says "I am preparing". Where is the word for "am"?

French does not have a separate present continuous tense like English does. The single verb form Je prépare translates to both "I prepare" (in general) and "I am preparing" (right now).