The DEFINITE ARTICLE indicates something very accurately. For example, the teacher asks the student to take "Il libro di grammatica" (the grammar book), not "Un libro" (a book).

In English you only have "The", pronounced in two different ways.

In Italian we have "IL, LO, LA, I, GLI, LE".

"LO" and "LA" lose their vowel and replace it with an apostrophe when they precede a name starting with a vowel, i.e. "L'aula" instead of "La aula".

Definite articles are used with titles: il signore, la signora, il professore, il dottore,...

BUT they are not used to address someone directly:

i.e.: Guarda la signora Anna = Look at Mrs Anna.

Buongiorno, signora Anna = Good morning Mrs Anna.

No! Don't worry, it's easier than  you might think and if you learn this rule well then you'll automatically learn many other important rules of the Italian language, but I'll explain you step by step when you are my student.

Il cane di Mario è nero = Mario's dog is black

Gli studenti di Valeria sono bravi = Valeria's students are good