In Italian articles and prepositions very often are combined. This is nothing really astonishing, that happens in a lot of languages.
Spanish: de + el = del => La puerta del coche. (the door of the car)
French: de + le = du => La jambe du kangourou. (The leg of the kangaroo)
German: von + dem = vom => Ein Stück vom Kuchen. (A piece of the cake)
The only difference between Italian and other languages is the fact, that in Italian much more prepositions are combined with the articles and due to the fact that the articles themselves have a lot of different forms, there are a lot of combinations.
But before we talk about these combinations we will shortly define what is a preposition. A preposition establishes a relationship between one object (which can be a noun or a pronoun) and another.
The book lies on the table.
The book lies under the table.
Contrary to his statement, he hadn' t paid.
Despite the bad whether, he had come.
Before dinner, he looks television.
After school, he goes swimming.
At first glance all that seems to be very easy and the role of a preposition in the sentence is quite obvious. We will see in the chapter about conjunctions that sometimes the difference between a preposition and a conjunction is not so clear and sometimes the same word can be a preposition, a conjunction and even an adverb. But for the moment the only thing we need to know is that these
a = to in = in di = from (genitive) da = from (origin, source) su = on
are prepositions and that they combine with an article.
In order to understand the table below we must remember how the definite articles il / lo / l' / gli / le are used, since the same rules explained in chapter 3.1.1 forms of the definite article, applies here. Therefore the following combinations are possible.
preposition / article
Il libro si trova sulla tavola.
The book is on the table.
Do una mela ai bambini.
I give an apple to the children.
Nei giardini ci sono dei fiori.
There are flowers in the garden.
La bicicletta sta nella cantina. The bicycle is in the garage.
Mi vedo nello specchio.
I see myself in the mirror.
Mi vedo negli specchi.
I see myself in the mirrors.