If you speak French you know that in this language exists a strange kind of article which exists only in few languages, the partitive article. One of the few languages with a partitive article beside French is Italian. Let's have a look at these sentences.
a) I want coffee.
b) I like coffee.
The thousand dollar question now is this: What is the difference between the coffee in sentence one and the coffee in sentence two. The difference is that: If someone asks you 'do you want tea or coffee' you can answer 'I want coffee'. But the meaning is, obviously, that you only want a little, very little part of the worldwide production of coffee. Case b) is different. If you say that you like coffee you are talking about coffee in general, you are not talking about a small amount of coffee. The partitive article is used if we talk about a part of something (Do you want some chocolate) or about some elements of a group of homogeneous elements (There are some cars in the street). You use the partitive article when you use some or any in English.
Do you have any questions? Hai delle domande?
Do you want some apples? Vuoi delle mele?
Do you want some sugar? Vuoi dello zucchero?
Voglio del caffè.
I like some coffee.
Mi piace il caffè.
I like coffee.
In the case of 'coffee' we have a noun that doesn't consist of single, countable elements, but something that can be imagined only as an entity. But the partitive article is used as well when talking about a limited number of elements from a group of similar elements.
Ho comprato dei fiori.
I bought some flowers.
Mi piacciono i fiori.
I like flowers.
The articolo partitivo (partitive article) is formed with the preposition di + article. As you already know, the genitive is formed the same way. In other words, a genitive looks like a partitive article, but is something really different from a grammatical point of view.
La porta della casa.
The door of the house.
Ho bevuto della spremuta d' arancia.
I have drunken some orange juice.
As you can see the genitive looks like an partitive article. It' s not very important to be able to distinguish between a genitive and a partitive article from a practical point of view, but sometimes it can be useful.
If you have studied French for a long time you know perhaps that there are some exceptions to the general rule (general rule: 'the partitive article is used, wenn we talk about a part of something or about some elements of a group with similar elements').
Ho comprato del pane.
I have bought some bread.
The table below shows the exception from the general rule. The most important one is b). The partitive article is not used in negative sentences. Better than trying to memorize the rules is to click on the loudspeaker on the right several times. It seems that the human brain is not very strong in memorizing abstract rules, but it is very strong in memorizing sentences. If you have one correct sentence you will form thousands of sentences the same way.
a) The partitive article is used when we talk about a part of something that can be thought of only as an entity (liquids, gas etc.) or if we talk about some elements of a group of similar elements (some pens from a group of pens).
Ho bevuto dell' acqua.
I have drunken some water.
Ho comprato dei coltelli.
I have bought some knives.
b) but is not use in negative sentences
Voglio mangiare delle patate.
I want to eat some potatoes.
Non voglio mangiare patate. (NOT: Non voglio mangiare delle....)
I don't want to eat potatoes.
not in certain idiomatic expressions
Non devi avere paura NOT: Non devi avera della paura
You must not be afraid.
c) not after prepositions
Senza amici la vita non è bella. (NOT: Senza degli amici....)
Without friends live is not nice.
Dopo settimane di attesa non so che fare. (NOT: Dopo delle settimane...)
After weeks of waiting I don' t know what to do.
Last not least it should be mentioned that the partitive article, although there is no difference to the French partitive article, is not used so often than in French, because in Italian there is tendency to substitute it with an adverb.
Voglio dello zucchero. <=> Voglio un po' di zucchero.
I want some sugar. <= > I want a little bit of sugar.