The partitive article seems to be a problem for everyone who starts learning French. The problem comes from the following three expressions:

J'aime le pain. (le) - I like bread.

Tous les matins, je mange du pain. (du) - I eat bread every morning.

Je voudrais un morceau de pain. (de) - I'd like a slice of bread.

1. In the first case, we use the definite article le because we refer to pain (bread) in general, and we do not need it to be physically present, all we need is to visualize it in our mind.

2. In the second case, we refer to some indefinite quantity of bread that needs to be physically present for us to perfom this action.

3. In the third case, we refer to a specific quantity - a slice of bread. We use the preposition de with expressions of measurements, weights and quantity instead of partitive articles.

Je voudrais un kilo de poivrons. - I'd like one kilogram of peppers.

Combien de lait je dois acheter? - How much milk do I need to buy?

In negative sentences, the partitive article is de (d'). De (d') also replaces the indefinite article in negative sentences.

Nous n'avons pas de pain. - We don't have any bread.

Nous n'avons pas d' eau gazeuse. - We don't have any carbonated water.

Tu as une voiture? Non, je n'ai pas de voiture.- Do you have a car? - No, I don't have a car.

If the verb is être, the partitive article is used in its full form in negative sentences.

C'est de l'eau gazeuse? Non, ce n'est pas de l'eau gazeuse. C'est de l'eau distillée. - Is that carbonated water? - No, this is not carbonated water. This is distilled water.